It is up to you --- either way hes going to go thru relocation stress ---- what was in the 40 gallon tank? You need to sanitize it real good w/ some F 10 you can find on Walmart.com its spendy but very good to keep on hand its a concentrate -- or you can find some Rescue on Amazon or Zoo Meds Wipe Out --- you want to kill what ever is in that tank -