My 4 year old has decided he wants to use his piggy bank money to get a fish tank and get a “rainbow betta” whatever that means. I’d like to get him a 10-20 gallon and would like to seed it with bacteria from one of my tanks but I have some questions. I have a sponge filter in my 5 gallon and plan to use a sponge in his as well. Would it make more sense and be best for my fish to buy the sponge now and let it sit in my tank for a while? If so how long before a new filter has enough bacteria to seed a new tank? Or should I put the new filter in my tank and use my current sponge in his tank since it’s been in use for several months? Any advise would be so appreciated.