Couple of things-

Fish excrete nitrogen in the form of raw ammonia, which is way too hot for your plants, esp after evaporative concentration in your media. The filter medium is designed to host several specia of bacteria, one of which converts the ammonium to nitrite- Still on the hot side. Another specia convert the nitrite to nitrate, which is acceptable as a nitrogen source.
Tank water contains all three of these forms of nitrogen in varying porportions, depending on filter load, tank pH, time of last feeding, etc. While a carefully managed system can produce a nute that won't actually kill your plants, the output of the system day to day is very difficult to track- you'll have no idea what your relative nitrogen concentration is, and what form it is in, unless you want to spend a lot of money on lab gear, and a lot of time learning how to perform the tests and a lot more learning to interpret them.

Almost all commercial fish food contains salt. Fish tanks accumulate sodium at a really rediculous pace. Fish are more tolerant of NaCl levels than your plants are. As the tank in question matures, the sodium levels will render the solution toxic. Again, this can be managed, if you have a half hour or so a day for labwork. And a lab.

"Fishtank dechlorinator" is sodium thiophosphate, which binds to the chlorine to produce, you got it, sodium chloride- salt. Again, sodium levels are too high for plants.

Most fish require a pH of between 6.5 and 7.2, which is too high for your purposes. This will cause micronutrient lockout, esp iron. You can "fix" this with a foliar feed, but the irregular availability of metals will inhibit growth.

If you want to hit your plants w/ tank water maybe once a month as a special treat, ya probabley won't kill anything. Probabley. They will respond to the nitrite spike by growing hard for a few days, assuming light is available. I wouldn't do it more than that , tho. In fact, I wouldn't do it at all.

Keep in mind that most " freshwater" tanks will show an EC of 1.0 or more- not what I'd call fresh. Unless you're really religous about water changes, it'll be higher. Most fishkeepers end up keeping the tank on the edge of toxic for the fish, forget about the plants- the tank gets bad, you clear it up until fish stop dying, you slack off, fish start dying, you clear it up....

So yes, tank water can be used, if you happen to have a good lab in the basement. And at least a Bachelor's in organic chemistry. And a lot of time.

Save yourself 10k in lab gear and another 80k in education- use tapwater.