Seven mistakes that kill fish in new tanks

Most beginner losses come from the same short list.

1. Adding fish to an uncycled tank. Without filter bacteria, ammonia builds up within days.
2. Buying too many at once. The filter bacteria need time to catch up with each new group.
3. Overfeeding. Uneaten food rots into ammonia. Feed what they finish in two minutes.
4. Replacing all the filter media at once. That throws away the bacteria. Rinse it in old tank water instead.
5. Mixing fish that need different water. Goldfish want it cool, bettas want it warm. Neither thrives in between.
6. Skipping the lid. Bettas, danios and loaches all jump.
7. Believing a fish only grows to the size of its tank. A cramped fish is stunted, and stunted fish die young.

The planner catches several of these for you. It flags temperature mismatches, tanks that are too small and schools that are too few.