brine is water saturated with salt (NaCl). salt is bad for tropical fish. i wouldnt give tuna in brine to a cat or dog either.
if u dont like toast then put it on bread with a garden salad and make a sandwich (an idea for next time, or the remaining tin/s, as it sounds like uve already used some of it)
if sox and bugsy are a cat or a dog, a once off shouldnt leave any lasting detriment to their health, but i would refrain from feeding them any more tuna in brine (tuna in spring water should be fine though)