I have an african dwarf frog and 6 Tiger barbs. The male tiger went up to the frog the first time he saw him and did the fishie equivalent of "sniffing" him - and lost interest. This may have been helped by the fact the the frog took this opportunity to swim off and kicked the male tiger in the face!
In all honesty, I have had no problems with the combination at all. We have 6 tigers (1 male and 5 females) and Spike (the male tiger) is so busy with his women that he really has no interest in anything else.
I know Tigers have a reputation of being aggressive but my experience is only if they need to be - Spike and his ladies are not even vaguely interested in anything other than themselves.