Medicating an aquarium safely: real volume, carbon and oxygen

Quick answer

More aquarium fish are killed by treatment than by the disease that prompted it. The three mistakes that do most of the damage are dosing to the tank's advertised size rather than the water actually in it, leaving activated carbon in the filter so the medication is stripped out and the dose is wasted, and forgetting that most treatments reduce dissolved oxygen at exactly the moment the fish need it most.

Before any of that, there is a prior question: what are you treating? Most of what looks like disease in a home aquarium is a water quality problem wearing a costume. Testing the water is faster, cheaper and safer than guessing at a medication.

Nothing goes into the tank until you know the real water volume and have tested the water. That order is the whole of safe treatment.

• Work out the true water volume, which is always less than the tank's nominal size.
• Remove activated carbon and other chemical media before dosing, and put fresh carbon in afterwards to strip the residue.
• Increase aeration and surface movement for the whole treatment period.
• Never combine two medications, and never overlap the end of one with the start of another without a full carbon strip and water change in between.
• Copper is permanent in a tank with substrate and rock, and it kills shrimp, snails and other invertebrates outright.

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Do not medicate on a guess.

Antibiotics dosed into the water kill the nitrifying bacteria in the filter as well as the target organism, which produces an ammonia spike in a tank full of already sick fish. Copper treatments bind irreversibly into substrate, rock and silicone and leach back out for years afterwards, so a tank treated with copper can never safely house shrimp or snails again. Formalin and malachite green are hazardous to the person handling them as well as to scaleless fish. If you do not have a diagnosis, the correct first move is a water test and a water change, not a bottle.
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Decide in 60 seconds

Getting the volume right

This is the single most common cause of accidental overdose and it is entirely avoidable.

A tank sold as a given capacity is measured to the very top of the glass. The water line sits some way below the rim, so you lose that band immediately. Substrate takes a further share, and a deep planted substrate takes a lot. Rock, wood, ornaments and equipment all displace water.

The result in a typical furnished aquarium is a real volume noticeably below the number on the box, and in a heavily aquascaped tank the gap can be large.

How to measure it properly, once.

Do it when you set the tank up or at a major maintenance. Empty the tank, then refill it with a container of known volume, counting the containers until you reach the normal water line with all the substrate and decor in place. Write the number on a label stuck to the cabinet or the back of the tank.

Or calculate and subtract.

Measure the internal length, width and depth to the water line. Multiply them. Then subtract a realistic allowance for substrate and hardscape based on the depth of substrate and how much rock is in there. This is less accurate than filling but far better than the box figure.

Then dose to the measured number.

If a product's instructions are given per unit of water, they mean the water present, not the tank's name.

The filter, the carbon and the biology

Remove chemical media before you dose.

Activated carbon, resin blocks and adsorbent pads all strip medications from the water. Leaving them in means the fish get a fraction of the intended dose, the treatment fails, and you conclude the product does not work when in fact it never reached the fish. Take them out, and store them dry or discard them.

Keep the mechanical and biological media in.

Sponges, ceramic media and the bacterial population living on them should stay. You are removing the chemical media only.

Expect the biological filter to suffer if you use antibacterial treatments.

Nitrifying bacteria are bacteria. Any antibacterial treatment in the water column affects them. Test ammonia and nitrite daily throughout an antibacterial course and be ready to do water changes, which then means redosing to maintain concentration. This is exactly why a separate hospital tank is worth the effort.

Put fresh carbon in when the course finishes.

Running fresh carbon for a period after the last dose removes the residue and shortens the period the fish spend in a medicated environment. Follow it with water changes.

A hospital tank needs very little: a bare bottom, a heater, a sponge filter seeded from the main system, a lid, and somewhere for the fish to hide.

Oxygen is the thing people forget

Fish in warm, medicated water are short of oxygen from three directions at once.

Warm water physically holds less dissolved oxygen than cool water. Many treatments call for raising the temperature, particularly against parasites with a temperature-dependent life cycle, and that immediately reduces the available oxygen while simultaneously raising the fish's metabolic demand.

Several medications consume oxygen chemically or interfere with gas exchange at the gill. Others irritate the gill surface directly.

And a fish that is already sick, particularly with a gill parasite or a gill infection, has reduced capacity to extract oxygen in the first place.

The fix is simple and should be automatic. Add an air stone or increase the return flow so the surface is visibly moving, and lower the water level slightly under a hang-on filter to increase splash if you need more. Do this before the first dose, not after the fish start gasping.

If you are raising the temperature, raise it gradually and stay well within the range every species in the tank tolerates. Cool-water species such as many goldfish varieties and some loaches will not tolerate the temperatures used for tropical parasite treatments.

Which fish and which tanks need extra caution

Invertebrates.

Shrimp, snails, crabs and corals are killed by copper at concentrations far below those used against fish parasites, and copper does not leave a tank readily once substrate and rock have absorbed it. Treat the fish elsewhere, or choose a product explicitly stated to be invertebrate safe, and even then test in a small way first.

Scaleless and small-scaled fish.

Loaches, most catfish, eels, elephant noses and stingrays absorb chemicals through the skin far more readily. Many medications carry a reduced dose recommendation for them, and salt is poorly tolerated by several.

Plants.

Some treatments, particularly certain algaecides and some copper products, damage or kill aquatic plants. Salt at therapeutic levels does the same.

Small tanks.

The smaller the volume, the more a small measuring error matters. Use a proper syringe or a graduated measure rather than a bottle cap.

Biofilter age.

A newly cycled tank has a fragile bacterial population and is the worst place to run an antibacterial course.

Fish that will be eaten.

Some treatments used in ornamental fishkeeping are not permitted for food fish and have withdrawal requirements. Never treat fish destined for the table with an ornamental product.

Running the course properly

Write the plan down before you start: measured volume, product, dose per unit, total dose, the interval, the number of doses, and the water change schedule.

Dose at the same time each day and mark it off. Missed and doubled doses come from memory rather than paper.

Do the water changes the instructions specify, and remember that removing water removes medication. If you change water mid-course, you generally need to replace the medication for the volume removed. If the instructions are silent on this, contact the manufacturer rather than guessing.

Complete the course even if the fish look better. Parasites with a resistant cyst stage in particular reappear if treatment stops when the visible stage disappears.

Do not switch products mid-course because you have run out of patience. Overlapping chemistries is a common way to kill an entire tank. If you need to change approach, do a large water change, run carbon, and give the tank time to clear first.

Keep a written record of what you used and when, including the batch. If a second problem appears months later, that record is genuinely useful.

Handling the products safely

Several traditional aquarium treatments are hazardous to people. Formalin is a preservative and a recognised carcinogen. Malachite green is toxic and stains skin and surfaces. Copper compounds are harmful if ingested.

Wear disposable gloves. Do not mix or measure over food preparation surfaces. Keep everything out of reach of children and pets, in original containers with the instructions attached. Wash your hands after any tank work, medicated or not, because aquarium water can carry organisms that cause skin and soft tissue infections in people, particularly through cuts.

Do not use a container that has held medication for anything else, and never siphon aquarium water by mouth.

What never to do

Do not medicate before testing the water.

Do not dose to the tank's advertised capacity.

Do not leave carbon or chemical media in the filter during treatment.

Do not combine two treatments, or add a second product because the first is working slowly.

Do not use copper in a tank containing invertebrates, or in any tank you might want invertebrates in later.

Do not raise the temperature without checking every species in the tank tolerates it, and without increasing aeration.

Do not stop the course when the visible signs disappear.

Do not treat a whole display tank for one fish's injury when moving that fish would do.

Do not use human medication, aquarium salt as a universal remedy, or a dose scaled from a different species.

Do not dispose of medicated water down a storm drain or into a garden pond.

Watch respiration and posture daily through a treatment. Faster gill movement, hanging at the surface or clamped fins means the treatment is now the problem.

The routine that prevents most of this

Quarantine every new fish, plant and invertebrate in a separate tank before it enters the display. This single habit prevents most of the outbreaks that lead to whole-tank medication.

Keep a hospital tank set up or ready to set up. A bare-bottom tank, a heater, a sponge filter kept running in the main system so it is always cycled, a lid and a piece of pipe for cover is all it needs.

Test the water on a schedule, not only when something looks wrong, so that you know what normal is for your tank.

Keep the true water volume written where you will see it.

Keep a written maintenance log with water change dates, test results and any additions.

Keep a fresh, unexpired test kit. Old reagents give false reassurance.

Know who you would ask. Aquatic veterinary services exist, and for koi, marine systems and valuable stock they are worth using rather than experimenting.

When to get help

Contact an aquatic veterinarian or an experienced fishkeeper before treating if the stock is valuable, if you keep koi or a marine system, if the tank contains invertebrates or corals, if fish are dying quickly despite good water parameters, or if a first course has failed.

Get help urgently if fish deteriorate within hours of a dose, if ammonia or nitrite rises during treatment, or if the whole tank starts gasping at the surface.

Ask for help before you buy anything if you cannot say what you are treating. A photograph of the affected fish and a set of current water test results is the minimum useful information, and a good aquatic vet or a specialist retailer can often narrow it down from that.

Bring or send clear photographs of the lesion or behaviour, a video if the problem is behavioural, current test results for ammonia, nitrite, nitrate, pH, temperature and hardness, the tank's true volume, the full stock list including invertebrates and plants, the maintenance history, and a list of everything already added and when.