Water changes
A water change does more than lower nitrate — that is simply the part you can measure. It also removes dissolved organic compounds no hobby kit tests for, replenishes calcium, magnesium and carbonate, and resets the slow drift in composition that leads to old tank syndrome.
In one sentence
Replacing part of the tank water dilutes nitrate and dissolved organics, replaces consumed minerals and restores carbonate buffer. It is the single most effective maintenance action available.
How much and how often
There is no universal schedule, because the right amount depends on stocking, feeding, planting and source water. Twenty-five to fifty per cent weekly suits most community tanks. What matters more than the exact figure is consistency: a regular moderate change is better than an occasional very large one.
| Change size | Proportion remaining | After two such changes |
|---|---|---|
| 10 % | 90 % | 81 % |
| 25 % | 75 % | 56 % |
| 50 % | 50 % | 25 % |
| 75 % | 25 % | 6 % |
Getting it right
- Match the temperature of the new water reasonably closely — within a degree or two is fine, a bucket of cold water is not.
- Treat for chlorine or chloramine before the water enters the tank, or dose the tank for the full volume as you refill.
- Vacuum the substrate in the open areas as you go; that is where most of the decomposing material sits.
- If a tank has been neglected for months, return it to normal over several days rather than in one enormous change.
Common misconceptions
| Often said | What is actually the case |
|---|---|
| Water changes remove the beneficial bacteria. | Almost all of the population lives on surfaces, not in the water. A water change removes very little of it. |
| A large water change shocks fish. | A large change of similar water does not. A large change of very different water — different temperature, very different pH or hardness — does. The problem is the difference, not the volume. |