My nephew gave me a 20 gallon hex tank with 5 mollies (all but 1 under an inch long) and a neon... so I bought 2 flying foxes (to keep tank clean) and 3 more neons because I heard neons did better in schools. However, now I have 11 fish in a 20 gallon with plants... I don't want them to start fighting. What's the rule of thumb on how many is too many?