so I've had a tank running for maybe 5 months now, I've got danios and minnows in a tank together with the temperature set at 21 degrees Celsius. My local fish speciality shop breeds the danios in cold water so they are actually able to thrive in this set up before anyone starts ripping into me for fish cruelty. however, my fish are dying and i have no idea why. ammonia and nitrites are both at 0ppm and nitrates around 20 ppm always, tap water is 8.2 but in the tank its around 7.2 (idk dont ask me how) water is crystal clear, i do weekly water changes of easily more than 20% ( i get carried away when vacuuming) i do seem to have diatoms which havent gone away (but only in places where i cant clean with my magnet scrubber). 1 fish seems to die every couple of weeks. first itll get moody and only stay in 1 place all day long. as a result, it wont eat any food. it gets skinny and dies of malnutrition. some of them got bent spines and sunken bellies but not all. sometimes they just die to spite me. i also have a minnow with a bent spine that is healthy and thriving and has been like this for a few weeks. right now, there are 2 minnows staying in the same spot all day up by the surface in a corner by the filter. they wont eat even when i basically hit them with the food. tank has just recently been wormed, fish shop doesnt have a clue. i dont have a clue. are these fish just genetic rejects or am i witnessing the come back of an ancient fish disease that nobody knows about with no specific set of symptoms lol. please help im over plucking minnows out of the filter grate with my fingers.