I have 4 freshwater prawns (Macrobracium) 2 are 2-3 inches, 1 is 1.5-2 inches, and the smallest is .5-1 inch. They attack and eat comet goldfish. Is there a fish that can be in this tank with them, that wont be eaten or eat them? Also is there anything that I need to do to the water in my tank to help them stay healthy? I have a 15 gallon tank, with one wisteria plant in it.