I am not sure whether you are overstocked. What are you parameters? When you do water change and then test it just before the next, is you NitrAte higher than the presvious week?
Sucking loaches, if they are what I think they are can get very aggressive as they get older and also quite large.
For guppies there should be at least 2 females to each male to minimise stress on the female.
Plecs, depending on type can get to 24" and they create a lot of waste. They need a very big volume of water, even for the smaller ones due to the waste they create.
Neons should be kept in groups of 6+. You're "unknown" tetra should be identified and will most likely need to be kept in a group.
Bettas should not be kept with fish with flowing fins such as male guppies.
What kind are your gouramis?
