If you have good flow/filtration, the poop should break down & get sucked up before you see it. I never see my 12" fahaka's poo.
Don't you wish you could do that for your dog, too?

I'd clean out the Eheim. I bet you'll find a TON of mulm in there. On my FW tanks, I use Aquaclear 110 filters for every 30-50g. Anything over 50g gets an Eheim added. My 125g has 3 AC 110s & an Eheim 2217. My 90g has 2 AC110s & an Eheim 2217. Once you move up to SW, you remove ALL FW filtra...

Are you watching him 24/7?

Yep, I'd be feeding an 8" puffer about every 3 days.

That'll work.

You've got enough room there for a single adult GSP. NONE of those other fish are BW species. I'm not surprised at your nitrate level. Lots & lots & lots of daily WC ahead. That blue lobster is pufferfood. Pointy snails? You mean Malaysian Trumpets? Way too hard for a puffer to chew on--can ...

3 days may not be long enough. Also, be very sure any ghost shrimp you offer him is fed 1st.

Wow, now those are some buck teeth! Looks as though he's quite thin from not eating enough. MS-222 is a powder. I'd use about 1/8 spoon in a gallon. If that doesn't knock him out, then double that. What tool will you use to clip those big teeth? Be sure to move him back & forth in the fresh tank...

Fatty!

Entire tank. Are you using Prime?

Why not just hook up a hose to the sink & adjust the temp before filling the tank?

Add a tab to the food, along with crushed garlic & soak for at least 1 hour.

Really great way to compromise your puffer's immune system, leaving it open for disease--especially, ich. This is why we float bags of new fish to match the temp. Better to fill your tank with a hose, matching temp.

I don't care what the directions say--IPs need to be treated INTERNALLY. FW fish don't drink water. That med will never touch it's innards if you just treat the water.

Did you ever receive the Finquel I sent you?