depends on the filters, but that sounds fine to me. just put the filters on to make sure, but 6 inches should be cool.....or just use canisters, the hoses are not that big and filters can go in the stand under the tank
go for around 8''-10 but just for power filters 6'' should be fine just where ever it need to be so you can fit behind it to clean or make it eazy to change filter pads keep in mind a tank that size will way over a ton so once its full you won't be moveing it till its empty
Ok, it appears I have to make more room at the back for it. 6 inchs wont be enough for cleaning maintenance on the back glass etc. I better go with 10 to 12 inches not sure yet. With 3/4 inch glass I dont ever plan on moving it empty even.