Copper in fish tank water: sources, testing and removal

Copper in a fish tank is toxic at very low levels, especially to shrimp, snails and other invertebrates, which can die from amounts too small for most fish to notice. If your shrimp or snails are suddenly dying, or you have just used a copper medicine, test for copper and start large water changes with copper-free water.

Copper usually gets in from old copper water pipes, copper-based fish medicines, some plant fertilizers, or metal objects such as coins and cheap ornaments. Once it is in, it can cling to silicone, substrate and decor and leak back out later, so getting rid of it takes patience.

How to identify it

You cannot see, smell or taste copper at harmful levels. Clues include:

Shrimp dying, often several at once, or failing to moult properly.
Snails dying or retreating into their shells and not coming out.
Fish stressed or dying suddenly at higher levels, especially in soft water.
Scaleless fish showing distress first among fish: loaches, and plecos and other catfish.
Problems that began after moving house, new plumbing, a medicine, a new fertilizer, or adding a new ornament.
A blue-green stain on taps or sinks, which can be a sign of corroding copper pipes.

The only way to know for sure is to test. Use a copper test kit on both your tank water and your tap water.

Why invertebrates and scaleless fish are so sensitive

Shrimp, crabs and snails use a copper-based blood pigment to carry oxygen, but they are extremely sensitive to extra free copper in the water. University fish-health guides say most invertebrates will not survive a copper treatment that fish tolerate. Scaleless and small-scaled fish, such as many catfish and loaches, are widely reported by keepers and medicine labels to react more strongly to copper, though the reasons are not well studied. Copper is also more toxic in soft, acidic water with low alkalinity (low carbonate buffering), which is the kind of water many shrimp and soft-water fish are kept in.

When it's a problem

Safe: No measurable copper. For shrimp tanks, aim for zero.
Concerning: Any measurable copper in a tank with shrimp or snails, or copper in your tap water.
Serious: Invertebrates are dying, or copper is present in a tank you did not dose.
Emergency: Fish gasping, lying on the bottom, or dying suddenly after a copper medicine or a suspected overdose. Act within the hour.

Exact "safe" numbers vary between sources. Many hobby sources report that sensitive shrimp can be harmed by levels as low as a few hundredths of a part per million. Copper-based fish medicines are dosed at many times that.

What to do right now

Stop adding copper. Stop any copper medicine, and stop using fertilizers or additives you have not checked.
Test tank water and tap water for copper. Also test ammonia and nitrite, because copper can harm filter bacteria.
Do a 50% water change with copper-free water. If your tap water contains copper, run the cold tap for a few minutes first, or use RO (reverse osmosis) water remineralised for your fish.
Add fresh activated carbon to the filter, or a copper-removing resin pad.
Move shrimp and snails to a copper-free tank or container if you can.
Remove obvious metal: coins, metal ornaments, brass or copper fittings, and anything painted or plated that is peeling.
Add an air stone if fish are breathing fast.

What to do next

Find the source

Old copper pipes. Water that has sat in copper pipes overnight, and hot water, carry the most copper. Soft or acidic water dissolves more. Always use cold water, let the tap run for a few minutes first, and test. Never fill a tank from the hot tap.
Copper-based medicines. Many ich, velvet and marine parasite treatments contain copper. Check the ingredients for copper sulfate or "chelated copper". Never use them where you keep shrimp or snails. See ich and velvet for alternatives.
Algaecides and pond products. Many contain copper.
Plant fertilizers. Plants need a trace of copper, and many fertilizers contain some. Most aquarium fertilizers labelled "shrimp safe" keep it very low. Garden and houseplant fertilizers can contain much more and should never go in a tank.
Coins, metal decor and fittings. Coins thrown into tanks and ponds, brass fittings, and cheap ornaments with metal under paint can leak copper or zinc.
Second-hand tanks, substrate and decor from a tank that was treated with copper.

Removing copper

Water changes: the main tool. Do 30–50% changes every day or two with copper-free water, testing as you go.
Activated carbon: absorbs copper. Replace it every few days while you clear the tank, because once full it stops working.
Copper-removing resin or pads: specialised media that grab metal ions. Very useful in shrimp tanks.
Chelated copper (copper bound to another chemical in some medicines) is harder for carbon to remove. Rely more on water changes.
Keep testing for several weeks. Copper bound to substrate, decor and silicone can leak back out, especially if the pH drops.

Copper that lingers

Copper binds to calcium-rich materials such as crushed coral, shells and limestone, and can be released later if the pH falls. Many experienced keepers also report that silicone sealant and some substrates hold copper and release it slowly. Experts don't fully agree on how long that lasts. If a tank has had copper medicine:

Replace the substrate, sponges and porous decor before adding invertebrates.
Scrub the glass and run fresh carbon for several weeks.
Test for zero copper more than once before adding shrimp.
Consider keeping a separate tank only for copper treatments, and never using it for invertebrates.

Cautions while you fix it

Match temperature and pH in new water so fish are not shocked. See pH.
Watch ammonia and nitrite. Copper can harm filter bacteria, causing a spike. See ammonia.
Plants usually recover, but some may melt back.

Products that help

Copper test kit: the only way to know if copper is present. Pick one that reads very low levels if you keep shrimp.
Activated carbon: absorbs copper and leftover medicine. Replace often.
Copper-removing resin or pads: targeted removal for sensitive tanks.
Reverse osmosis (RO) unit or bought RO water: a copper-free water source when tap water is a problem.
Remineraliser: adds back the minerals fish and shrimp need to RO water.
Dechlorinator: some also claim to bind heavy metals. Treat that as a short-term help, not a fix.

How to prevent it

Test your tap water for copper before setting up a shrimp or snail tank, and again if your plumbing changes.
Always use cold water, and run the tap first.
Read the ingredients of every medicine, fertilizer and additive.
Don't put coins, metal ornaments or unknown decor in the tank.
Be careful with second-hand tanks and substrate.
Treat sick fish in a hospital tank, never the shrimp tank.

Why it happened

Copper most often comes from corroding household pipes, especially with soft or acidic water and water that sits in pipes. The other common causes are copper medicines, copper-based algaecides, unsuitable fertilizers, and metal objects in the tank.

When to get help

Get help from an aquatic vet if fish are dying after a copper treatment. If your tap water has high copper, contact your water supplier, or get it tested if you are on a private well. Copper in drinking water can also affect people, especially babies. In the US, the EPA action level for copper in public drinking water is 1.3 mg/L.

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