It is simple- do 125 gal water change every other day and filtration system should be not important- even NO FILTRATION will be good enough. But remember- water for WC must be aged .
Hans
PS. I'm working on it. But must find one year old pictures and carefully consider all DISADVANTAGES of undergravel/jaubert filtration system. The major one is probably necessity of soft and acidic water. In tap water bacteria and fungi will eat your fish almost for sure.