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Brackish water is about 1/2 fresh water, and 1/2 marine water. Hard to explain in one post, why don't you do a bit of research on it. Cichlids will die in brackish water.
You need to do some serious research about general fish keeping. (Most) Freshwater fish can not be in brackish bc they are freshwater fish. Most puffers need a tank by themselves. You're right about your GSP needing brackish water to thrive but cichlids are freshwater fish - both Africans and new worlds. Regardless, and I don't want to sound rude, you sound very uneducated with the needs of your fish - please do some research on your fish and on any fish you're looking to buy in the future. Here to help