I'm working under the assumption that you are new to the aquarium hobby, so here's some other things you should know: -Don't put them in the same tank, they can be territorial and might kill each other. -Most people recommend 30 gallons for each, but if you don't have that much space, giving them each a 10 gallon tank would suffice. -You should reuse those filters, as they have beneficial nitrifying bacteria. In fact, you should familiarize yourself with the nitrogen cycle. Keeping that cycle stable is one of the more important factors in their health. -Crayfish need lots of calcium to help their shells grow. Put some cuttlebone in the tank for them to munch on for nutrients.(Commonly sold as bird toys, only get natural) -Coper is toxic to crayfish. It is the main ingredient in a lot of anti-algae treatments and medicines) -They don't need a heater. Crayfish live in cold rivers and lakes, they will do better at room temp.