How can you even find the little guy in a 40gal? He's got a mansion to live in. But hey by no means is that bad. Its good to see someone actually prepared for the future. About the filter, what kind is it? Power, Canister, Internal? Also how deep is your water? Hatchling musks need shallow water. Keeping them in deeper water is fine as long has you have a good sized shallow area where he can rest and not have to swim to get air. After I know what kind of filter you have, I will be able to help you out more.
Chris
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Hey whats up, I'm Chris and I currently have 1.0 Softy, 0.0.1 Midland Painted, 1.0 Bearded Dragon