Not really sure about that fish tank water, and what it really has in it. Even if the ph is ok, other stuff might not.

Better would be to use ph down (phosphoric acid for flowering plants, while nitric acid for plants in veg). People will sometimes use lemon juice for a quick fix, but shouldn't be used constantly. Try to use around 6.5ph water.

If you just watered yesterday I'd say it's OK to flush now. Flushing with 3x the water is standard procedure for nute burn, don't know how I missed that before (sorry bout that )

Keep in mind that it takes a while to notice the corrections you made in soil. The burnt leaves won't get better, so check the new growth. If you don't feel the need to flush, then don't. Maybe the plain water feedings are starting to help. We're not there so I think you can make best decision.