But I could put 20 neon tetras in a 20 gallon and be fine. Why? Because it goes by bioload
Neons are shoaling fish, come from streams. Having 10 in a 10 gallon is crap too.
The 1" is a rule is not a rule it's a GUIDELINE....can you say it with me...G U I D E L I N E...I did not want to elaborate, since the person only had small fish, but here goes.
The general stocking guide is 1" of fish for every 1 gallon of water. There are different modifications to the guide. For simplicity sake, we can modify this to apply to fish under 3". Larger fish can still be generally stocked at about 1" of fish for every (at the most) 5 gallons of water to accommodate some of the above. Please keep in mind that this is a general stocking guide and
NOT an exact science. It still does not take account other factors, such as:
- How much swimming room the fish will require (6 neon tetras will probably not be happy in a 10 gallon tank)
- Swimming level (top, mid, or bottom dwellers)
- The amount of waste a fish will produce
- Group amount (ex. Clown loaches would prefer to be in group of at least 3)
- Aggression levels
- Compatibility (ex. Clown loaches would not be happy with fast, darting fish)
- Tank Length/Height requirements (Angelfish require taller tanks, which neon tetras prefer longer tanks)
An improperly overstocked tank will lead to:
- Elevated ammonia
- Elevated nitrate
- Low oxygen levels
- Harassment from other fish
- Lack of hiding places
- Lack of enough fish to provide schooling
Unfortunately, these are similar causes in
stress.
A more accurate stocking guide can be found at
Alternative Stocking Rules for Freshwater Aquariums, which still leaves out some factors. This is where the person should take the responsibility of researching on the type of fish they would like to stock.
Again, I repeat...This is a STOCKING GUIDE, not a RULE. It is also not an EXACT SCIENCE.