I've been wondering this for some time now.
Is your aquarium/are your fish salt water? If so, would the salt water harm the plants?
Is your aquarium/are your fish "brackish" (part salt/part freshwater)?
Is your aquarium freshwater?
I have a 75gal "brackish tank" (African Cichlids) and add 3-4 tablespoons of salt every 4-5-6 weeks and have been wondering the same thing. Can I water with "my tank water"???
Have you looked into aquaponics? Do a search for it and see the cool shit ppl are doing with talapia which are a cyclid (?)(Mouth breeding, Black African Carp). That would be my best lead as far as the salt question. Salt is used for other things in the garden (root weevils on rhodies) so it sounds okay to me. I would try a side by side experiemnt.
In aquaponic systems, the effluent is circulated through a hydronponic set up to grow greens but not fruiting plants. The systems need a few micronutrients added but seem to be enough to crank out the lettuce and herbs.
If I had a fish tank that hadn't had toxic stuff added, I would for sure put it on my outdoor garden. I would not depend on it as the main nute and would suspect it would be best for veg stage and would not add the nitrogen during flowering.
BTW Tilapia are the fastest protien production in the animal world. They can produce a fish of 1 1/4 lbs, filleted down to 3/4 lb, with 1, count em, 1 lb of fish food. New advances have them eating only grass and spirulina (grown with the fish) so you have no ongoing food costs. A tank in the garage can feed the family and provide lots of fertilizer.