How much water to change by tank size
The usual starting point is about 25% once a week. Enter your tank, the share you want to change and the size of your bucket to see the gallons and the number of bucket trips.
Gallons by tank size and share
US gallons to replace, with the number of 5 gallon buckets in brackets. The sizes are the label volumes of common tanks.
| Tank | 10% | 25% | 30% | 50% |
|---|---|---|---|---|
| 10 gal | 1 (0.2) | 2.5 (0.5) | 3 (0.6) | 5 (1) |
| 20 gal | 2 (0.4) | 5 (1) | 6 (1.2) | 10 (2) |
| 29 gal | 2.9 (0.6) | 7.3 (1.5) | 8.7 (1.7) | 14.5 (2.9) |
| 40 gal | 4 (0.8) | 10 (2) | 12 (2.4) | 20 (4) |
| 55 gal | 5.5 (1.1) | 13.8 (2.8) | 16.5 (3.3) | 27.5 (5.5) |
| 75 gal | 7.5 (1.5) | 18.8 (3.8) | 22.5 (4.5) | 37.5 (7.5) |
Which share to pick
Practical Fishkeeping says about 25% once a week works well for most people, but it depends on your tank. It suggests comparing nitrate in the tank with nitrate in your tap water: a difference of more than 40 ppm means more water, more often.
My suggestion: start at 25%, test nitrate right before the change for a few weeks, and let the readings move the number. The schedule page turns a weekly nitrate rise into a percentage, and the nitrate page shows what repeated changes do to a high reading.
The math behind the buckets
Water to replace is volume × share. A 29 gallon tank at 25% is 29 × 0.25 = 7.25 gallons, which is one full 5 gallon bucket and 2.25 gallons in a second. Buckets are volume × share ÷ bucket size.
Use the water volume, which is lower than the label. Gravel, sand and rock take up room, so going by the label overstates the gallons a little. The tank volume calculator subtracts that space. Fill a bucket only to the line you measured, since a bucket's rated size is rarely what it holds below the rim.
Treat the new water before it goes in. The dechlorinator calculator gives the dose for the volume you just worked out, and the shrimp tank page covers tanks that need smaller, slower changes.