yes, it is moslty good unless the ph is different. good for jump starting new tank but might be dangerous for tank with fish in it since it will fluctuate the water chemistry (if it is different that is )
If your old fish died, get moved to a new tank, ect. Can you use the same water the old fish were in for a new one, like, I’m planning on getting a betta for my ten gallon once I move my tetras to my 47, could I use the same old water I used for my tetras for a new betta
hope this makes sense! I’m not planning on doing it, just wondering ?
I would always do a major water change, meaning 75-80% of the tank's volume, before adding new fish. You could drain it completely of course, since there are no fish in it. There is certainly no benefit in "old" water, it is full of "stuff" that we remove via water changes so the fish will be healthier.