My GSP is in a 10 gallon, this is just temporary, Ill be getting a big tank (minimum 50 Gallon) in the next month or so. I bought him before I had a big enough tank because I usually don't see too many GSP's for sale at the local pet stores, and hes still only 1.5' long, so its not too bad for a month....plus he's all by himself in there with a bunch of rock caves and his little ceramic pipe (his 'krib').
Im going to throw in a sacrificial feeder guppie this week to gauge how aggresive he will be towards other potential tank mates when he gets his new home.
If you have yours in a 10 gallon also, I hope you'll be getting a bigger tank soon as GSP's like their space, and they grow up to be 4-6" long, not to mention they apparently get more aggresive as they get older, so anything in a small confined space will be seen as food by the GSP.
Also, I owned cichlids before, and they get very aggresive when older as well, is the yellow lab in the same tank as the GSP?....I think you will have to consider getting another tank in the future.
Whats your PH, specific gravity, and temperature of your tank?