Tap water that’s perfectly fine for people is acutely toxic to fish.

This isn’t a mild concern or a precaution. Chlorine at normal tap concentrations damages fish gill tissue. Chloramine stays in the water until chemically neutralized and is toxic to fish even after the water has sat for 48 hours. The gap between what humans can drink and what fish can tolerate is large enough that treating aquarium water is non-negotiable.

Here’s what’s actually in your tap, what to do about it, and when standard dechlorinated tap is enough vs. when you actually need RO water.

Chlorine: The Baseline Problem

City water is disinfected with chlorine to kill bacteria and pathogens. At tap concentrations of 0.5-2 mg/L, chlorine is safe for people. For fish, that same concentration damages gill tissue and disrupts osmoregulation, the process fish use to regulate water and salt balance across their membranes.

Free chlorine does off-gas. If you fill a bucket of tap water and let it sit uncovered for 24-48 hours with some surface agitation, most free chlorine will dissipate. This was once a common practice for small tanks. It’s slow, not always complete, and doesn’t work for chloramines.

The faster, more reliable approach is a liquid dechlorinator. Sodium thiosulfate is the active ingredient in most basic options and neutralizes free chlorine immediately.

Chloramines: The Part Many Hobbyists Miss

About half of US water utilities use chloramines as their disinfectant, either instead of chlorine or in addition to it. Chloramine is a compound of chlorine and ammonia. It was adopted partly because it’s more stable than free chlorine, meaning it stays in the distribution system longer without breaking down.

That stability is exactly the problem for aquariums. Chloramine doesn’t off-gas when water sits out. It stays in the water indefinitely until chemically neutralized.

Sodium thiosulfate does not neutralize chloramines. It breaks the chlorine-ammonia bond, but that releases free ammonia into the water, which is toxic to fish. You need a complete dechlorinator that specifically states chloramine neutralization.

Seachem Prime is the industry-standard choice. It neutralizes both chlorine and chloramine and also detoxifies ammonia and nitrite temporarily. API Stress Coat and several others also handle chloramine.

Check your utility’s Consumer Confidence Report (mailed annually or available online) to find out whether they use chlorine, chloramines, or a combination. If the report isn’t clear, call the utility directly. Knowing this determines which dechlorinator you actually need.

Heavy Metals: Copper and Invertebrates

Copper is acutely toxic to invertebrates, shrimp, snails, and crabs at very low concentrations. Older homes with copper pipes can leach elevated copper into water, particularly first-draw water that’s been sitting in pipes overnight.

A dechlorinator that chelates heavy metals (Seachem Prime again, or API products with metal-binding claim) provides a buffer. For tanks with sensitive invertebrates, this matters. For a basic fish-only tank with no invertebrates, it’s a secondary concern.

If you have copper pipes and keep shrimp or reef animals, testing your tap water’s copper level is worth doing. A basic copper test kit costs $8-12.

pH and Hardness: Match the Fish, Not the Trend

Different fish have different water chemistry needs. This is where treating all tap water the same becomes a problem.

African cichlids prefer hard, alkaline water: pH 7.8-8.5, high general hardness (GH). Most American city tap water naturally falls in this range or close to it.

Most tetras, rasboras, and South American species prefer soft, acidic water: pH 6.0-7.0, low GH. If your tap water is alkaline and hard, these fish won’t thrive long-term even if it’s perfectly dechlorinated.

Discus and many dwarf cichlids are stricter still: very soft water, pH 6.0-6.5. Standard tap water almost never matches this without treatment.

Test your tap’s pH and GH before you buy fish. A basic liquid test kit covers both. Then research the requirements for the fish you want, not the other way around. It’s much easier to match fish to your water than to modify your water chemistry constantly.

When RO Is Actually Necessary

For most community freshwater tanks with adaptable species, properly dechlorinated tap water works fine. You don’t need RO for a typical setup with tetras, livebearers, guppies, or most easy cichlids.

RO becomes worth it in four situations:

Your tap water is very hard or very alkaline and you want to keep soft-water species. RO gives you near-zero TDS water you can remineralize to exact specifications.

You want a planted tank with CO2 injection and a low-pH environment. Controlling carbonate hardness (KH) is much easier starting from RO than working against your tap’s natural buffering.

You’re keeping a reef tank. Reef water chemistry is tight, and tap water contaminants like silicates, phosphates, and nitrates interfere with corals. RO with a DI stage is standard for saltwater.

Your tap water has tested high for specific contaminants, copper being the main one for fish-specific toxicity.

For everything else, a good dechlorinator and a basic parameter test is what you need.

Concrete Starting Point

Figure out whether your utility uses chloramine. Check your CCR or call the utility. If they use chloramine, buy Seachem Prime or another chloramine-rated dechlorinator. If they use only chlorine, any standard dechlorinator works.

Test your tap’s pH and GH with a liquid test kit. Pick fish appropriate for those parameters, or decide whether the effort of RO remineralization is worth the fish you want.

Start with dechlorinated tap and the right fish for your water. That works for most setups, costs nothing beyond the dechlorinator, and doesn’t require any extra equipment.

Add RO when your fish’s specific needs actually require it.