What Happens If You Skip the Dechlorinator

Written 2026-10-19

Skip the dechlorinator on a water change and the chlorine in tap water damages your filter bacteria colony a little bit, right there, every single time. Running a 60cm standard tank with 10 neon tetras in Water Kiwoogi, with a weekly 25% water change starting day 1: with dechlorinator, cycling finishes by day 23 and all 10 fish survive. With the exact same schedule but no dechlorinator, cycling doesn’t finish until day 76, and one fish dies on day 24 along the way.

What a dechlorinator actually does — it’s not activated carbon

Tap water carries leftover chlorine (or bound chlorine, chloramine) added at the treatment plant for disinfection. It’s safe for people to drink, but it’s toxic to the nitrifying bacteria living in your filter — the colonies that turn ammonia into nitrite and nitrite into nitrate. A dechlorinator neutralizes that chlorine chemically; it’s a different job from activated carbon, which filters out odor and color. Every time you mix in fresh tap water without one, the colony takes a hit sized to how much water you just swapped in.

Skip it and cycling takes three times longer

The same tank (60cm standard, mid-size canister filter, 10 neon tetras, weekly water changes from day 1) was run with only dechlorination and water-change size changed. With dechlorinator, cycling finishes by day 23 and all 10 fish make it. Without it, at the same weekly 25%, the colony keeps taking damage and cycling doesn’t finish until day 76 — one fish dies on day 24 along the way (9 of 10 survive). Push the water-change size to 40% with no dechlorinator over 150 days, and cycling barely finishes by day 148, with two fish dying on days 22 and 24 (8 of 10 survive) — that early-death risk overlaps with what’s covered in "Why fish die within a week in a new tank."

Even a tank that looks settled keeps relapsing if you keep skipping it

Another comparison: starting weekly 25% water changes only after cycling has already finished, on day 29. With dechlorinator, the cycling that finished on day 24 simply stays finished. Keeping the same schedule without dechlorinator for 150 days doesn’t look dramatic on paper — the peak readings only reach 0.03ppm ammonia and 1.05ppm nitrite, barely above the 0.01ppm and 1.05ppm you’d see with dechlorinator. But because the colony takes a small hit every single week, the tank doesn’t actually settle all the way back down to zero until day 111 — 87 days after it already looked done.

So dechlorinate every water change, not just sometimes

How much damage the colony takes is proportional to how much water you swap in that time. One accidental skip matters far less than making it a habit — skip it regularly during cycling and the finish date stretches out for weeks while fish die in the gap; skip it regularly on a tank that already looks stable and the colony can’t fully grow in for months, so ammonia and nitrite never quite settle at a clean zero. The upside is that dechlorinator is cheap and the dose per change is small, so doing it every time barely costs anything. It’s worth plugging in your own tank’s size, filter, and water-change schedule and toggling dechlorination on and off to see the difference yourself. How long cycling normally takes is covered in "How many days does cycling take."

The button below runs the 60cm standard tank, mid-size canister filter, 10 neon tetras, weekly 25% water change from day 1 (no dechlorinator, 90 days) tank from this guide.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.