Only problem is did you say 20 gallon? They require minimum of 30 gallons. fluctuation in a 20 gallon SW tank are great and the GSP wont do well with fluctuations. Also they have those beaks to feed on shells so clean up crew with them is not an option. actually you should be feeding them snails and shrimp to keep the beak filed down. Corals I have never tried so I don't know. Up your tank to a 45 and you will have one happy puffer I had mine in a 45 with damsels and it did well.
Good luck to you they are great little fish with huger personalities, by far one of my favorites.
Even though they have the beaks to break the shells if snails, I don't believe they're strong enough to break the shells of astreas and turbos. They have extremely hard shells that are tough to break. Mine has been living with snails and hermits and it tried to eat them at first but the shells are so tough that the gsp couldn't break them and just gave up entirely. I'm going 5 years and those snails and hermits are still in there kicking it.
I completely missed the part about you putting it in a 20. I'd say that's fine for a small 1 but you need to keep track of the params. Don't skimp on topping off either cuz even though a drop in salinity is fine for them, a rise is a different story.
Where are you located? If you're local or close to me, I can sell you mine, that way you don't have to go through the hassle of converting it. I'm just trying to find the time to ship my fimbriated moray to a member and when that's done, I'm tearin the tank down.
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