Please help. I have an 182l tank which currently houses 6 danios, 1 very large shubunkin a comet and 2 small fan tails. My issue is they are constantly getting sick. I was cleaning the tank once a week replacing 30% water but was told that was too much now I only replace 20% once every 2 weeks... My fish seem to get sick one right after the other constantly since I got this tank... is it something I'm doing wrong? I run a test kit each week too and my nitrate and ammonia levels etc are fine... please help I cant keep quarantining fish or adding salt every week... I only had these issues when I upgraded my aquarium never before with my old tank.