I have a 20 gallon tank ,tall, that has 5 zebra danios, a pleco, a betta, and about 9 guppies. I am thinking about returning the pleco and keeping only 4 male guppies. how many zebra danios and neon tetras can I have with the betta amd 4 guppies in there? My zebra danios are nippy fish they keep nipping my guppys. =[ It has about 13 live plants. I have my temperature at 83-84 degrees so I can cure the ich outbreak from my neon tetra that ALL died out.(When it was still in the nitrogen cycle I think) I don't have a water parameter test kit. I need to get one. Would keeping the heater next to the HOB Powerfilter really help circulate the water so that the water at both sides will be equal?