Saw sick gsp in pet store and would like to save.

[DerpyFish]

I saw a skinny gsp stuck to the filter in the pet store yesterday. I really want to bring it home and try to rescue it but I don't have a brackish tank set up. All I have is a twenty gallon high and possibly some freshwater filter media I could try to start the cycle with if that would even work. I know the twenty gallon won't be big enough long term. How long before it will need a bigger tank? Should I just give up now? It was in bad shape but seems like a fighter.
I'm also worried that I might have got him over fed yesterday. The guy working there didn't even realize there was still a puffer in that tank and when he saw how skinny he was gave him a bunch of food. I didn't help. I was egging him on cus I was worried about the fish. Today the owner acted like what we did might kill him but he's still alive so far. He was very dark when we found him but as soon as he realized he was getting food he brightened up. Also, the sunken in place is more on one side. It's almost like his tail is bent, but he still swims okay. The last fish I had with a bent tail never swam right. I'm bad about falling for the sick ones. I'm also bad about falling back on my maintenance. If I'm going to do this I'll have to stick with it.
So I guess I'd just really like you guys thoughts on where to go from here. I already bought a stand and an extra filter and some driftwood and salt mix. I guess I'm pretty well committed now. How do I set up a brackish tank? How do i mix the water? Will the freshwater media help start the cycle at all? Can I start with a Molly? I don't have access to pure ammonia and I would like to get started ASAP. Can I use fake plants for hiding? Will any real plants live? Should I get some rocks? He probably has parasites. How do I treat them? What do I feed him? How much? Just all the general new puffer fish owner questions and sick fish questions on top of that. Thanks.

[Pufferpunk]

Let it go, it's almost dead. You'd have to force-feed with a tube to save it. By "rescuing" a sickly, un-cared for fish, you are only perpetuating the cycle of mistreatment by a store like that. That one will sell, die in your care & they'll by more. Let THEM take the loss. Maybe they'll stop selling them someday. Go buy a fat healthy one from a reputable shop that cares.

[Pufferpunk]

http://www.thepufferforum.com/forum/lib ... d-puffers/
http://www.thepufferforum.com/forum/lib ... encycycle/
http://www.thepufferforum.com/forum/vie ... =8&t=19348

[pokeystar]

Chances are the fish is currently in freshwater in the shop. If so starting him in freshwater with the established freshwater media and some safe start should be ok. Lots of water changes and clean out uneaten food. You can add salt slowly using a hydrometer to bring the salt levels up over time. If hes eating treat for Internal parasites almost immediately.

[DerpyFish]

I was thinking of doing that but he is being kept in brackish. Would it hurt him if I cycle the tank freshwater and drip acclimate him to it? Or would it be better to start the tank brackish?

[DerpyFish]

He is in brackish water at the store. Should I try to match it or would he acclimate to freshwater? What's the quickest way to cycle the tank and get him into it?