Hi. I have a 55 gallon tank that I completedly tore down, cleaned and set up again. It was set up for over a month before I started introducing fish. I started with 3 med. size parrot fish. Then angelfish. There are 4 med. size koi angels that are doing great. Every time I add smaller angels, they do great for a few days, then they stop eating, then they die. I've been through 13 angels now. I take care to acclimate them slowly. I've thoroughly checked all the water parameters and it is fine. I did initially have hard water causing the PH to rebound but have started using a water softner pillow with my buffer and it has helped. Considering an RO unit as we have well water. Your thoughts?