Okay, so I am very new to aquariums and it is actually only my second week keeping fish. At the minute I feel completely useless as all my tetra are dropping like flies. My tank is properly cycled, is 29gal, I bought the full works second hand within a few hours of it being disconnected and all my water parameters are good: ammonia 0 nitrite 0 nitrate 50 (tap water reads at 38) PH 7.6 Currently in my tank are a couple of guppies, a bristlenose, and some cardinal tetra. Tap water is treated with tetra aquasafe. Now, the tetra were the first fish i bought, they were happy, schooled, lost a couple on the second day which i put down to moving stress. The cardinals no longer school, they hang around in a corner, look healthy and happy, swimming close to their group. I presumed they are maybe nervous of the guppies with their crazy behaviour. Anyway, nearly everyday im taking out dead tetra, no symptoms of illness just dying overnight...water remains fine. At this point im down to 3 cardinals and I have put it down to them not liking the water, as our water is quite hard. This morning one of the tetra had a torn fin, probably from the guppies as one of them is quite rough, but now a few hours later there is a white growth around the fin...i also suspect ich on the cardinals but not on the guppies. I am fairly sure that the cardinals are all going to be gone within the next few days, im not sure how i failed them but everyone else seems happy. What i want to know now is, does this fish have fin rot from the tear, or is it something else, most of all, is it contagious? I do not have a quarantine tank as i do not have the room. Hope this made sense and was not too much rambling haha.