Shrimp and snails in a fish tank: copper, and the treatments that wipe out invertebrates

Quick answer

Everything on this table can go into the water. In a tank with shrimp or snails, the jars and pouches are the ones that need reading before the lid comes off.

Shrimp and snails are not simply small fish. They are arthropods and molluscs, and several substances that fish tolerate easily are lethal to them at a fraction of the dose.

The commonest way a shrimp colony or a snail population dies is not disease. It is the keeper treating the fish, dosing a plant fertiliser, or using an insecticide in the room, without knowing that invertebrates sit on a completely different sensitivity scale.

• Copper is the classic killer. Read every ingredient list for copper sulphate, chelated copper or cupric ion before dosing anything.
• Copper does not break down. It binds to substrate, rock and silicone and leaches back for months, so a tank that has been treated with it may never be safe for invertebrates again.
• Some dewormers used for fish are lethal to snails specifically. Fenbendazole and its relatives are the usual cause of an unexplained overnight snail wipe-out.
• Household insecticides and spot-on flea treatments target arthropod nervous systems. Shrimp are arthropods.
• Deaths are often delayed and cluster around moulting, so a treatment given on Monday can kill on Thursday.

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Never dose a copper-containing medication into a tank you intend to keep invertebrates in, now or later.

Copper disrupts ion regulation across invertebrate gills, and crustaceans and molluscs are affected at concentrations that leave fish unharmed. There is no rinse that undoes it: copper adsorbs to gravel, rock, wood and the silicone seams, then releases slowly back into the water for months afterwards.

If the fish need copper, the fish move to a hospital tank. The invertebrates never move into the treated one.
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Decide in 60 seconds

Why invertebrates sit on a different scale

Fish and invertebrates fail in different places, which is why one dose can be therapeutic for one and lethal for the other.

Copper's main effect on crustaceans and molluscs is on the gill. It interferes with the ion pumps that maintain salt and water balance, so the animal loses the ability to regulate its internal chemistry. It also binds to respiratory proteins. Crustacean and mollusc blood carries oxygen using haemocyanin, which is copper-based, and free copper ions in the water disturb systems that fish, using iron-based haemoglobin, are simply less exposed to.

Size and surface area make it worse. A shrimp is small, its gills sit close to the water, and its whole body has a high surface to volume ratio. Whatever is dissolved in the water reaches its tissues faster than it reaches a fish's.

Then there is moulting, which explains the timing that confuses so many keepers. A moulting shrimp takes up water and minerals rapidly to split and expand its new shell. Anything dissolved in that water goes in with it. A concentration that a shrimp survives while its shell is intact can kill it during the hours around a moult, which is why deaths from a Monday dose appear on Thursday, and why they hit some animals and not others.

Insecticides are the clearest case of all. Pyrethroids, neonicotinoids and similar compounds are designed to attack arthropod nervous systems. Shrimp are arthropods. There is no dose that separates them from an insect, and quantities small enough to arrive as aerosol drift across a room are enough.

Snails mislead in a different way. Their response to hostile water is to withdraw into the shell and seal. An owner sees closed snails, waits for them to come out, and by the time it is clear they will not, the exposure is days old.

What actually gets into the water

Fish medications.

Ich and velvet treatments frequently contain copper, either as copper sulphate or in a chelated form intended to stay dissolved for longer. Chelated does not mean invertebrate-safe, it means longer-lasting.

Malachite green and formalin combinations are hard on invertebrates and on scaleless fish. Some products combine them with copper.

Anti-parasite dewormers vary enormously. Praziquantel and metronidazole are generally tolerated by invertebrates. Fenbendazole and related benzimidazoles are lethal to snails, and this is the usual explanation when every snail in a planted tank dies within a couple of days of a plant treatment or a worm treatment. They are also used deliberately to kill hydra and planaria, which is exactly why they end up in shrimp tanks by accident.

Levamisole is used against planaria at carefully controlled doses and has a narrow margin in shrimp tanks.

Algaecides and snail eliminators.

Most snail-killing products are copper-based, and most algaecides marketed for ponds and tanks are either copper-based or work by a mechanism that also strips oxygen. Neither belongs in an invertebrate tank.

Plant fertilisers.

Copper appears in comprehensive micronutrient mixes because plants need trace amounts. Dosed to label in a normally stocked tank this is generally tolerated. Doubling the dose to fix a deficiency, or dosing the full amount into a heavily invertebrate-stocked nano tank, is a known cause of losses. Shrimp-specific fertiliser ranges exist for this reason.

Tap water.

Copper plumbing releases copper into standing water. The first draw in the morning, or the water in a newly installed copper pipe, carries more than water that has been running for a while. Hot water systems and older fittings add to it. Running the tap before filling, and using a conditioner that binds heavy metals, both help.

Household products.

Insecticide sprays, fly killers, plug-in vaporisers, air fresheners, oven cleaner, perfume and aerosol deodorant all reach the water surface, and an air pump drawing from a treated room pumps them straight in.

Your hands.

Spot-on flea and tick treatments for cats and dogs are potent invertebrate toxins. So are some hand creams and soap residues. Rinse to the elbow with plain water before putting an arm in a tank, and never work in a tank on the day you have applied a spot-on treatment.

Objects.

Coins, brass fittings, copper pipe used as decor, treated wood and painted ornaments all leach.

Reading the label properly

Front-of-bottle claims are marketing. The ingredient list is the document.

Look specifically for: copper sulphate, cupric sulphate, chelated copper, copper gluconate, cupric ion, malachite green, formalin or formaldehyde, fenbendazole, flubendazole, levamisole, permethrin, pyrethrin, deltamethrin, and emamectin.

Be careful with the phrase "safe for the biological filter". That is a claim about nitrifying bacteria, not about shrimp.

Be equally careful with "reef safe", which refers to marine corals and their symbionts and does not automatically transfer to freshwater shrimp.

Where a label gives no ingredient list at all, treat the product as unsuitable for an invertebrate tank. There is no way to make a judgement about something that will not tell you what it contains.

When the fish genuinely need treating

A fish that needs medicating goes to a hospital tank. The display stays clean, and the invertebrates and the biofilter both survive the treatment.

The rule is simple: move the patient, not the invertebrates.

Set up a bare hospital tank with water taken from the display, a heater, an airstone and a seeded sponge filter. Treat there, at the correct dose for the actual water volume in that container, with no carbon in the filter while dosing.

Do not attempt the reverse. Catching every shrimp and snail out of a planted tank is impossible, snails are already in the substrate and on the filter, and shrimplets are invisible until they are not.

If the display itself must be treated, for example for a parasite with a free-swimming stage that lives in the substrate, accept that this is a decision about the invertebrates. Choose a treatment with an invertebrate-safe active ingredient if one exists for the disease, and if it does not, move the invertebrates to a temporary tank and expect the display to remain unsuitable for them for a long time afterwards.

After any accidental exposure, a series of large water changes, fresh activated carbon and a copper-adsorbing resin will bring levels down. Copper test kits exist and are worth buying if you keep invertebrates. Where the exposure was substantial, replacing the substrate is the only reliable fix.

Telling poisoning from the other causes of sudden invertebrate death

The last row is the most useful diagnostic in the table. A problem that kills the invertebrates and leaves the fish looking well is almost always chemical and almost always something you added.

Running a mixed tank safely

What never to do

Never dose a copper-containing product into a tank you want invertebrates in, including one you plan to add them to later.

Never rely on the phrase "invertebrate safe" without an ingredient list to check it against.

Never use a snail eliminator in a tank with shrimp. Almost all of them are copper-based, and shrimp are more sensitive than the snails being targeted.

Never assume a dewormer is harmless because it is sold for fish. The benzimidazole group is used deliberately to kill invertebrates.

Never spray insecticide, air freshener or cleaning product in a room with an open-topped tank or a running air pump.

Never top up an aquarium with hot tap water. It carries more dissolved metal from the plumbing than cold water does.

Never dose the display tank "just in case". Prophylactic medication of a healthy tank is the single most common route by which a keeper poisons their own invertebrates.

Never move shrimp into an old tank that has been medicated in the past without testing it and, ideally, replacing the substrate.

When to get help

Invertebrate losses are usually solved with a test kit and a careful account of what entered the water, rather than at a clinic. Write down everything added to the tank or used in the room in the previous two weeks, including plant treatments, new decor, cleaning products and any spot-on applied to another pet in the house.

Contact an aquatic vet or an experienced specialist retailer the same day if fish are affected as well as invertebrates, if losses continue after large water changes and carbon, or if a whole population died within hours of each other.

Bring the tank volume and age, the full stocking list, ammonia, nitrite, nitrate, GH, KH, TDS and temperature readings, a copper reading if you have a kit, the exact product labels including the ingredient panels, and the dates and doses used.

If several tanks in the same household are affected, look at what they share: the same water source, the same bucket, the same net, or the same room air. That is almost always where the answer is.