Converting FW to BW.. for GSP

[electromen]

Hi. I'm considering converting my 46g tall from FW to brackish... I want to change the substrate as well..
Is there any way of doing this without having to start the cycle from scratch?

Ideally, I'd like to house a GSP in the tank. I'm assuming it would only be for that guy, but I'll ask it anyways... What else could I house in that tank?
What would be the best substrate for a GSP? Eventually I want to go full marine with it.

I have some urothphlamus fry, from what I read, they can tolerate, and in some cases, prefer brackish conditions. But would they be an ideal companion for a GSP?

Any help is appreciated.

[Pufferpunk]

If there are little to no fish in there, you won't have enough bacteria to support a GSP--especially if you remove the substrste. I'd go ahead & replace that with crushed coral (or aragonite sand but I prefer CC for easier cleaning). Most GSPs are sold in FW in shops. Either fishless cycle or add Bio-Spira at the time of purchase & add at the same time as the puffer for an instant cycle. Then you can raise the SG .002.week, as you do water changes. GSPs do not make good tank mates, especially as they mature.
http://www.thepufferforum.com/forum/library/puffers-in-focus/an-introduction-to-green-spotted-puffers/
There are many other good puffer articles in that Library.

[electromen]

Thanks for your reply.
Its an established tank right now. The LFS that has the GSP has them in freshwater. Could I just remove the substrate, replace it with either CC or aragonite, put my established filter back on and wait for a mini-cycle? Then add the GSP and slowly convert it to brackish after that?
That sounds like the most logical thing to me. I could use the uro's to trigger the mini-cycle as they are pretty hardy before I send them off to the buyer.

[Pufferpunk]

If you leave the tank fallow for more that 24 hours, the biological bacteria will start dying off. To keep it alive, you should be feeding it ammonia, by either using plain ammonia or just toss in a some flake food.