This was on my 55g goldfish tank. I drained the water down to about 25% and had it refilled to 80-90% before I realized I never added Prime before refilling!
I quickly added a heavy dose when I realized I forgot. So far I've seen no ill-effects. The fish are as happy and active as they always are and the nitrogen cycle is still doing it's thing. I just did another measure for ammonia. 0ppm.
On Sundays I do WCs on my Oscar tank (75g) and goldfish tank. They're on a double top/bottom stand, so it's usually pretty quick and easy. I was all tired and cranky being in the middle of a some 4pm-4am shifts, had gotten up about noon and was doing these WCs before heading back to work. This was the first and hopefully last time I forget to dose dechlorinator!

On Sundays I do WCs on my Oscar tank (75g) and goldfish tank. They're on a double top/bottom stand, so it's usually pretty quick and easy. I was all tired and cranky being in the middle of a some 4pm-4am shifts, had gotten up about noon and was doing these WCs before heading back to work. This was the first and hopefully last time I forget to dose dechlorinator!