I don' think using " since " in here would be grammatically correct Since means " from a time in the past until a later past time " Your sentence is general , so you should use simple present tense " to talk about a fact that was true before, and is still now, and will be in the future " So my suggestion is "The best way for a pair of Cichlid Fish to breed is to keep them together once they are born " Once/ at the time / at the moment
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