Shrimp tank filtration: sponge filters, cycling, and the intake that kills shrimplets

Quick answer

A nano shrimp tank. The part that decides whether the colony lives is the least visible one: the bacteria living inside the filter sponge.

A sponge filter is the default filter for a shrimp tank for two reasons that have nothing to do with cost. It has no impeller and no narrow intake slot, so it cannot shred a shrimplet, and its foam holds an enormous bacterial surface in a small volume, which is what a nano tank is short of.

The filter is not the plastic. It is the bacterial film growing inside the foam. A brand new sponge is not a filter at all, and stocking shrimp behind one is the single most common way a first colony dies.

• Buy the sponge, then spend weeks growing bacteria in it before any shrimp arrive.
• Put a sponge pre-filter over the intake of any other pump in the tank, before the first shrimplets hatch, not after.
• Rinse the sponge in water taken out of the tank. Never under the tap.
• Never replace a sponge with a fresh one. Replacing it throws the filter away and keeps the housing.
• Put the air pump above the water line, or fit a check valve, or a power cut will siphon the tank onto the floor.

:::danger
A new sponge filter is a piece of foam, not a filter.

Nitrifying bacteria have to colonise it, and they multiply slowly. Until they do, ammonia from the shrimp and their food stays in the water. Shrimp show almost nothing while this happens: they simply stop moulting cleanly, then die over a few days, usually blamed on a bad batch.

Nothing about the equipment prevents this. Only time and a tested ammonia and nitrite reading of zero do.
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Decide in 60 seconds

Why a sponge filter, and how it actually works

An air pump pushes bubbles into the lift tube at the centre of the sponge. The bubbles rise, and rising bubbles drag water up with them. That draws replacement water in through the foam from all sides.

Two things happen as water passes through. Particles are trapped mechanically, and dissolved ammonia is consumed by bacteria growing on the wet pore walls.

Those bacteria are the entire point. One group oxidises ammonia to nitrite, a second oxidises nitrite to nitrate. Both are slow growing compared with the bacteria that cause a cloudy bloom in a new tank, which is why cycling takes weeks rather than days.

Foam is used because of its surface to volume ratio. A block of open cell foam the size of a fist carries far more colonisable area than the same volume of gravel, and every square millimetre of it sits in a slow, oxygenated flow, which is exactly what nitrifiers need.

The gentleness is a consequence of the design rather than a feature added to it. There is no motor, so the only force in the system is the lift generated by bubbles, and that force is spread across the whole outer surface of the sponge instead of being concentrated at a slot.

For shrimp this matters at two life stages. Newly hatched shrimplets are barely larger than a grain of sand and cannot swim against a current. Freshly moulted adults are soft, slow and hide for hours, and any strong suction near a hiding place is a trap.

There is a second benefit that no other filter type gives. The outside of a mature sponge grows the same brown and green biofilm that grows on wood and leaf litter. Shrimp graze it continuously, and shrimplets in particular rarely stray far from it. A sponge filter in a shrimp tank is furniture, filter and pantry at once.

What a working setup looks like

Shrimp graze every surface in the tank continuously. A mature sponge adds one more grazing surface, and shrimplets tend to stay on it.

The parts are an air pump, airline tubing, a check valve, and the sponge assembly itself. Some pumps benefit from an air control valve so that a single strong pump can run two tanks.

Bubble rate should be set low. The visible test is the water surface: a slow rolling movement is right, a rattling boil is not. A violent output does not filter faster, because the bacteria are already saturated with the ammonia the water carries. It only strips carbon dioxide, evaporates water, throws salt creep around and makes shrimp avoid that end of the tank.

Position the sponge where it will not be buried. Shrimp tanks accumulate plant mass fast, and a sponge grown over by moss is a sponge with no through flow.

Check valves are not optional in most installations. An air pump below the water level holds a column of water in the airline whenever it stops. When power returns, most pumps push through it, but during the outage that column can siphon the tank down to the level of the airline outlet and out onto the floor. A one way check valve, fitted the right way round, prevents it. Fitting one backwards blocks the air entirely, which is worth checking on day one rather than during a power cut.

For a tank with a fish or two as well as shrimp, a sponge on its own is often enough. Where more mechanical clarity is wanted, run a small hang-on-back alongside it with a sponge pre-filter pushed onto its intake pipe.

Sizing it, and the numbers that do not matter

Filter ratings printed on the box refer to tank volume, and volume is the wrong variable. A nano tank fed sparsely produces a fraction of the waste of the same tank fed heavily, and it is waste that the bacteria have to process.

The useful way to size a sponge is by how much you feed and how many animals are eating. Shrimp are small and produce little, but shrimp colonies grow, and a colony that has doubled twice is a different bioload from the ten animals you started with.

Aquarists commonly buy a sponge rated for at least double the tank volume, on the reasoning that a bigger sponge takes longer to clog and holds a larger bacterial reserve. That rule of thumb is sound for the reason behind it rather than for the number.

Two smaller sponges are better than one larger one in any tank you intend to keep long term. They give you a seeded sponge to move into a quarantine tank or a new setup, and they let you clean one at a time so that you never disturb the whole filter at once.

A matten filter, which is a slab of foam dividing off one end of the tank, is the same principle at a larger scale, and it suits shrimp very well. The trade off is that it takes up tank length, which a nano tank does not have much of.

The intake that kills shrimplets

Shrimplets do not appear gradually. A berried female carries eggs under her tail for weeks, and then one morning there are twenty animals in the tank that were not there the night before.

By that point every intake in the tank must already be protected, because the first thing shrimplets do is disperse.

A hang-on-back or canister intake pulls water through a slot into a pipe and then into an impeller chamber. Adults are usually pinned against the slot rather than drawn in, which is survivable if you notice quickly. Shrimplets pass straight through and are killed by the impeller.

The fix is a foam pre-filter that pushes onto the intake pipe, sold for the purpose in several sizes. Fine mesh over the slot works too but clogs far faster and starves the pump.

Powerheads, wavemakers and internal filters have the same problem and the same fix. Heaters do not, but heater guards are worth having anyway because a shrimp that settles on the hot face of an unguarded heater can be cooked.

An overflow to a sump is the hardest case, because shrimp will go over the weir. Foam or a mesh screen over the weir teeth is the usual answer, and it needs cleaning on a schedule since it will clog.

Cycling the sponge before the shrimp arrive

Growing the bacterial film is the whole job, and the tank cannot be rushed through it.

The bacteria need three things: a surface, a supply of ammonia, and oxygen. The sponge gives the first and the air pump the third. The ammonia has to be supplied deliberately, either as a measured ammonia solution or by letting a small pinch of food decay in the tank.

Test ammonia and nitrite as the weeks pass. The pattern is ammonia rising then falling as the first group establishes, then nitrite rising and falling as the second catches up, then both reading zero with nitrate present. Only when ammonia and nitrite both read zero, and stay there for several days after another dose of ammonia, is the filter real.

The fastest legitimate shortcut is borrowed bacteria. A squeezed out sponge from an established, healthy tank, or better, a whole second sponge that has been running in someone else's mature tank for a month, transfers a working colony directly. Transport it wet, in tank water, and install it the same day. Bacteria in a sponge that has dried out or gone anoxic in a sealed bag are dead.

Bottled bacteria products vary enormously in whether they contain live organisms of the right species by the time they reach you. Treating one as a head start rather than as an instant cycle is the safe assumption.

Shrimp are added last, slowly, and drip acclimated. Shrimp are far more sensitive to abrupt changes in dissolved mineral content than most fish, and a colony moved straight from bag water into different water can moult badly within a day.

:::info
Water changes do not reset a cycled tank, because the bacteria live on surfaces rather than in the water column. Changing water removes nitrate and refreshes minerals. It is cleaning the sponge in chlorinated water that removes the filter.
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Changes that mean act today

Temperature is part of filtration. Warmer water holds less oxygen and drives every biological rate in the tank faster, including the rate at which waste is produced.

Bubble output has dropped, or the sponge looks swollen and dark.

The pores are blocked. Through flow has fallen, and the interior of the sponge is losing oxygen. Squeeze it out in removed tank water now rather than at the next scheduled clean.

Shrimp gathered at the surface, or all clustered at the outflow.

That is an oxygen sign, not a filtration sign. Check temperature first, since warm water holds less oxygen, then increase surface movement.

Any ammonia or nitrite reading above zero in a stocked tank.

Stop feeding entirely for a few days, since food is the ammonia source. Small water changes matched for temperature and mineral content. Do not clean the filter while this is happening: the bacteria you need are in it.

Shrimp dying one or two at a time over a week, with no obvious signs.

Look for the invisible causes before disease. Copper is the classic one, from plant fertiliser, from a fish medication used in the same tank, from tap water standing in copper pipes overnight. Failed moults are the second. A filter problem shows up as poor water tests; a copper problem usually does not show up in tests at all.

Water suddenly cloudy and white a few days after a clean.

A bacterial bloom, caused by disturbing the filter or by an ammonia surge. It clears on its own. Do not treat it, and do not clean the filter again to fix it.

The routine that keeps a sponge working

A written log matters more here than in a larger tank. Nano volumes swing fast, and the useful information is the trend across three weeks, not the reading on the day something goes wrong.

What never to do

Never rinse a sponge under a running tap. Chlorine and chloramine in mains water are added specifically to kill bacteria, and they do it in seconds. This is the single most common way a keeper crashes a working tank while trying to be conscientious.

Never replace the sponge with a new one. If a sponge is genuinely falling apart, add the new one alongside it and run both for at least a month before removing the old one.

Never clean the filter and change a large volume of water on the same day. Each disturbs the system separately, and doing both at once is how an apparently stable tank produces an ammonia reading the next morning.

Never use filter media or pads marketed as antibacterial, and never put copper based algaecide, snail treatment or fish medication into a shrimp tank. Invertebrates are far more sensitive to copper than fish are, and much of what is sold to treat fish is copper based.

Never run a sponge filter dry during maintenance. If the tank is being emptied for a rescape, keep the sponge submerged in tank water and keep an airstone in that container.

Never assume a filter that looks clean is working. Sponges look identical whether they hold a mature bacterial colony or nothing at all. Only a test kit tells you.

When to get help

Shrimp sharing a tank with fish. Every pump intake in a mixed tank needs a foam pre-filter fitted before the first shrimplets appear, not after.

Aquatic veterinarians who see invertebrates are uncommon, and much of the useful diagnostic work in a shrimp tank is done by the keeper with a test kit. Escalate when the pattern does not fit water chemistry.

Get advice the same day if several shrimp die within hours of each other, if any shrimp is found dead with an intact but empty looking shell and white banding at the join behind the head, or if losses continue after ammonia, nitrite, nitrate, GH, KH and TDS have all been confirmed normal.

Sudden simultaneous losses point to something added to the water: a treatment, a fertiliser, an aerosol used in the room, an untreated top up, or a new piece of hardscape.

Bring this information to an aquatic vet, an experienced shrimp keeper or a specialist retailer: tank volume, how long it has been running, the exact filter and how it is cleaned, full test results including TDS and GH, the water change routine including what the replacement water is made from, everything added in the last month including plants and fertiliser, the temperature range across the day, and photographs of a fresh casualty.

That history identifies the cause far more often than examining an individual shrimp does.