i do everyday or every 2 days water change to copy the natural replenishing of streams and rivers, well, i really like my sharks, i know they need schools, but i did have 6 of them, but 4 succumb to ich(it was my fault, i didnt know the warning signs of ich, and i didnt know how to treat it, so i was too late). I might get rid of the danios, or tranfer the sharks into my other 200 gallon, and get a few more...