Since it already has 2 in it take one out and put a new one in then in about a month you can replace the other one but never replace the bio wheels.well its been 2 months since my 29 gallon has been running so i decided to change it
nd i got a question my penguin biowheel filter has 2 filter cartridge spots so when i want to change the cartridge should i justput the new cartridge in the 2nd slot and after like a week take out the old one?