Most first tanks die because of the water, not the fish. Tap water carries chlorine or chloramine, both of which burn gills and also kill the bacteria in the filter that keep the tank alive.
The options:
- A dechlorinator, which works instantly and handles chloramine too
- Standing water in an open bucket for 24 hours, which lets chlorine gas off but does nothing at all for chloramine
- Boiling, which is impractical at volume
- Bottled or RO water, expensive and stripped of minerals
Which of these do you use? And the questions nobody can answer generically:
- Does your area supply chlorine or chloramine? Has anybody actually asked their water board?
- How hard is your tap water, and what did you have to change because of it?
- Boring water in from a tanker -- does that change what you do?
If you have a test kit and some numbers, post them with your city.