Is Tap Water Safe for Isopods?

For a plain isopod colony, UK tap water straight from the cold tap is almost always fine. The dose of disinfectant in it is small, isopods are not sensitive to it at the levels involved, and the water is going into damp substrate rather than being drunk.

That is the honest short answer. The longer answer matters if you keep springtails alongside them, or if you have been told to leave water standing overnight, because that advice only works on some UK supplies.

Why the standing water trick is half wrong

UK water companies disinfect with chlorine, chloramine, or both, and which one you get depends on where you live. The amounts are small, usually a few tenths of a milligram per litre and under 1, against World Health Organisation guideline maximums of 5 for free chlorine and 3 for monochloramine. There is no legal ceiling in the UK: companies set the level high enough to keep the water disinfected and low enough to be acceptable to drink.

Free chlorine fades. It goes over hours rather than minutes: the water companies' own tip for losing the taste is a jug in the fridge for a few hours or overnight, and fishkeepers leave water uncovered for a day or more. That is the advice everyone repeats, and on a chlorinated supply it works.

Chloramine does not. It is chlorine bonded to ammonia, it is deliberately more stable so it survives longer in the pipes, and it will still be there tomorrow. Standing water does nothing to it. Boiling is not a reliable route either. Removing it takes a catalytic activated carbon filter, or an aquarium water conditioner that says on the label it treats chloramine as well as chlorine.

So if you have been standing water overnight and feeling careful about it, you may have been doing nothing at all. Whether that matters is the next question.

Finding out which one you have

Your water company publishes an annual water quality report, and it may well say. Search your supplier's name and "water quality report" with your postcode, and if the report does not name the disinfectant, a call to the company will settle it. It takes a few minutes and it is the only way to know rather than guess.

Do not assume chlorine because that is what the old advice was written for. About a third of Scottish Water's customers get chloramine, Thames Water uses it mainly in London and Yorkshire Water uses it too, while Welsh Water, Bristol Water and Southern Water add none.

When it actually matters

For isopods alone, it does not. At UK concentrations, in damp substrate, in an enclosure that is being aired, this is not what kills colonies.

For springtails, be more careful. They are smaller, they sit directly in the moisture film, and a springtail culture is far more sensitive to a knock than an isopod colony is. If you are misting a culture you rely on, use rainwater, bottled water, or tap water treated with a conditioner labelled for chloramine.

The same applies to a bioactive setup you are trying to establish. The point of a bioactive substrate is the microfauna and bacteria in it, and those are more sensitive than the animals you can see. Once it is established it will shrug off tap water; while it is getting going, give it the easier option.

The one water that is dangerous

Never use artificially softened water. Domestic softeners work by ion exchange: they take out calcium and magnesium and put sodium in. That gives you water that is high in salt and stripped of exactly the mineral an isopod needs for its exoskeleton.

If you have a softener plumbed in, most installations leave the kitchen cold tap on the hard supply for drinking. That is the tap to use. If you are not sure which taps are softened, find out before you mist anything.

Hard water is fine, and mildly useful

Hard water is calcium and magnesium carbonate, and neither is a problem. Isopods spend their lives looking for calcium. Limescale on the glass is cosmetic.

It does not replace a proper calcium source. Cuttlebone or crushed oyster shell in the tub does a job that misting never will, but there is no reason to soften or filter hard water for invertebrates.

Do not put in a water dish

This is the more common mistake, and it is nothing to do with water quality.

Isopods take their moisture from damp substrate, from food, and from the air. A dish of standing water gives them nothing they need and is a drowning hazard for mancae and small species. If you want to raise humidity, dampen one end of the substrate or add wet sphagnum, not a dish.

Sponges and cotton wool in a dish are worse rather than better. They stay wet, they go sour, and they grow things you did not intend.

Water quantity beats water quality, every time

Colonies do not usually fail because of what is in the water. They fail because there is too much of it and not enough airflow.

A sodden, stagnant tub grows mould, invites grain mites and stalls breeding. Run a gradient instead: damp at one end, dry at the other, and let the animals choose. Mist the damp end when it starts to dry, not on a schedule.

If you are worrying about chloramine while your substrate is waterlogged, you are solving the wrong problem.

Frequently asked questions

Can I use tap water to mist my isopods?

Yes, in almost all cases. UK tap water is lightly disinfected and isopods tolerate it at those levels. Use rainwater, or tap water treated with a conditioner labelled for chloramine, if you are also keeping springtails or establishing a new bioactive substrate.

Does leaving tap water out overnight make it safe?

Only if your supply is chlorinated. If it is chloraminated, standing does nothing, and a fair share of UK supplies are, from parts of London to a third of Scottish Water's customers. Check your water company's annual report to find out which you have.

Is bottled or filtered water better?

Marginally, and rarely worth the cost for isopods. Rainwater collected in a clean butt is free and does the same job. Do not use distilled or reverse osmosis water as the only source for a colony you are also failing to give calcium to; it contains nothing at all.

My water is very hard. Should I do anything about it?

No. Hard water is calcium and magnesium, and neither harms invertebrates. Do not run it through a domestic softener, which swaps the calcium for sodium.

Do isopods need a water dish?

No, and it is safer without one. Moisture comes from the substrate and from food. A dish is a drowning risk for mancae and does nothing a damp end will not do better.