Actually, fw fish aren't' a good alternative food source. Lions are not particular and will eat just about anything so, yeah, they'll take it but, long term it could cause health problems. Also, I believe goldfish are extra bad and will cause them to get fatty liver which will shorten their life. Here is an excerpt taken from B. Fenners book...
"Goldfish are not a good steady diet for several reasons. They're nutritionally deficient, incovienient, expensive and my make your lion aggressive."
I feed mine every other day and I alternate food from frozen/thrawed shrimp, silversides and freeze dried krill. You can pick up a few bits of fresh fish or shrimp from your local grocer for pennies and be giving him a much better diet.