Marine/Freshwater?
Fish Crazy
Ive got a Rekord 120 brackish set up with green spotted puffers. Brackish is no harder than tropical, you just literally add a few spoons of salt. I use AquaTonic Salt, and add 1 tablespoon for every 10-12 litres. So when you do a water change (10%), just add another tablespoon of salt, and thats it! Obv watch for salt creep when the water evaporates and so increases the salt level, but a cheap hydrometer from your LFS will do the trick. Easy!!
Actually Aquatonic salt is not the right salt! It is for freshwater tanks. You need salt that is used in marine tanks. Your tank would be ok for figure eights but GSPs need a salinity around 1.008 at least as they mature. If you have more than one in 120 liters they will get agressive real quik. At six inches they will get really cramped.