Help!!! Forgot Dechlorinator

The fish should be OK, and the filter bacteria too, but keep an eye on your levels just in case.

Overdosing the last bucket was the right thing to do. But for future reference - add enough dechlorinator to treat the whole tank, not just the new water, as soon as you realise you've forgotten. You could always go and add a bit extra now, just in case.
There is a reason for this. Fish, and plants to some extent, secrete organic chemicals. You'll read of things called DOCs in tank water - that's dissolved organic chemicals, those things secreted by the fish. DOCs react with dechlorinator so it then can't remove the chlorine. So if you add untreated water you need to add more dechlorinator so there's enough to remove the chlorine.

People with really big tanks use hose pipes to refill after a water change, and they obviously can't dechlorinate the water before it goes in. They use enough dechlorinator to treat the whole tank, adding half before they start and half after they've finished.

Ive just done the same thing Mervyn! 

I merrily replaced two buckets of untreated water to the tank. It was when I reached for the dechlorinator on the third bucket that I realised I hadnt treated the first two. Time to panic.... so I poured an unmeasured amount into the third bucket and did the same straight into the tank.

Im now kicking myself something chronic for being so stupid in the first place (never forgotten dechlorinator before so no idea where my head was today) and also now panicking that Ive overdosed with the stuff. Consequently also kicking myself something chronic for not measuring the dechlorinator when I realised Id forgotten it, rather than just pour a load in  If it wasnt for the fact that I have to stand my tapwater 24hrs before using it, I'd be in the middle of a 90% water change right now!

On a bright note, I have just googled overdosing and have learnt that it would take an awful lot of dechlorinator to cause problems. They say that its tested at 100x recommended dose with no ill effects. Even so, my tank is getting an extra large water change next week, if not before!

If its any consolation to you, your mistake was not in vain! It was because of this thread that I knew what to do..... though Ive no excuse for not measuring!