I am not experienced growing marijuana, YET, but I'm already working on transplanting my fourth batch of seedlings, all to finish up this year. I burned some plants with fish emulsion, so I can tell you it's possible. You have to go with 50% more than the recommended dosing, that's all it takes. It STINKS, to high heaven. I wish I'd just purchased the liquid seaweed extract earlier. I swear, I think I'm gonna try cooking with that stuff, it smells like some excellent Asian flavoring sauce!
Don't use fish emulsion as a foliar feed unless your goal is to attract flies and make it impossible to sleep (our bedroom is what's on the other side of my garden, click my gallery and you'll see the setup).
I think it requires gentle use, a strong stomach, a stronger nose, and I'll probably use it for the trees that are FAR from the house instead. If mixed properly and used quickly, it's a great way to quickly get nitrogen to the plants, if that's what is needed. It didn't work as quickly as I'd hoped, though.
If your plants are growing, are not showing yellowing or other issues, then leave them be is what I say. In other words, if it ain't broke, yeah? You could scratch in some guano or worm castings, a type of quickie top dressing if you're really worried about it. I would honestly suggest you do a search on Ohsogreen's posts, the guy gives very explicit instructions and explains a lot of stuff along the way, fantastic (and he got me a small freebie from Blue Mountain Organics, the makers of Super Plant Tonic that I'm gonna be trying out starting today).
Aquarium water is something I WISH I had. It's great for all sorts of plants. It's been filtered biologically, has fish poop that's also been broken down, and tends to be soft unless you're keeping fishes that seriously need hard water (such as certain types of African rift lake cichlids) and you've set up the system to meet those. Most tropical freshwater fish that are kept, though, do well in all types of water, and many, especially New World cichlids, prefer softer, acidic water, which just happens to be what our mary jane likes.
I was just talking to my husband the other day about a bin I'm using to store my filtered water that is also combined with what little rainwater I could gather and how I'd like to drop in some crawdaddies. We get food for ourselves, and food for the plants.