so...I have a 75 gallon tank with a heater maintained at a temp of 75 degrees F a solid 7 PH, 0ppm ammonia, 35 nitrate and those were tested a week ago. I have 6 long finned zebra dannios, 6 bleeding heart tets, 5 male guppies, and a partridge in a pear tree! Tee hee. This is obviously a freshwater tropical tank that I also keep live plants in. Hornwort, grasses, amazon sword plant, and cocomba. The gravel is light colored an I have high lighting in the tank with two rock caves as well. Plenty of free swimming areas and hiding places. So now to my question...what is the largest fish I can put in the tank with the guppies. I'm having trouble with PH compatibilty and other issues like brackish water fish and others. I've looked at the amiet killifish and giant dannios/devarios, but I just am not sure what to do. I have about 46 in of fish I can add. I do regular changes of water(reverse osmosis) and I treat my plants with fertilzer( the brand escapes me). Let me know what you think...I need variety because of the tank size but I dont want to sacrifice my itty bitty fish. A little more color wouldn't hurt either. I also have a 10 gal that I use as a hospital tank.