I just got finished adding a sump, just wondering if that means i can have another fish. or will it still be over stocked? its a 20 gal with a 4 gal sump?

well after working alot of kinks out with filter and overflow suction, i have now added 3 zebra danios, 2 serpae tertas ( already had 1), and a chinese algae eater . lost a lot of fish to my filter strainer and overflow strainer. now all seems well tho.