The fact that you don't test the water is a HUGE thing.
Do you know what cycling a tank is? Do you know about water parameters such as ammonia, nitrites, nitrates, pH?
These are a HUGE part of fish keeping. If ammonia is above .25 it can kill your fish and if there's ANY trace of nitrite it can do the same. Look into the API freshwater master test kit you can get it on Amazon for 15 dollars right now (there's a sale going on)
Do you do water changes? How often?
I would get that test kit ASAP you're fish will not be healthy as they can be without it. If there's a nitrite or ammonia or even a pH spike, your fish will get unhealthy real fast and die off.
I may be a little TOO obsessed with my angels...