r/MonsterFish

I have a two foot long tiger shovel nose red tail hybrid catfish and a week ago I noticed a big gash on his side, I’ve been doing 25% water changes every day but now he’s now sitting on the bottom of the tank and not very active, I have polyguard and serchem focus on hand and I’m transferring him into my 150 (my temporary hospital tank) with no decor and low flow to reduce stress. Any suggestions to help him??? I know it’s super risky to treat scaleless fish but he hasn’t been improving….