"what put it in a bucket or something? like one that hasent been used for chemicals of course. "
That's the one. I bought a couple of 10-litre buckets, with graded measurements, for fishtank use only, one for clean and one for dirty water. (then last week, I decided this process needed speeding up, so invested in 2 more buckets, only 1.99 each and fit into one another so don't take up more space).
You use the gravel vac to get out the dirty water. I pur it down the loo or down the drain as I don't fancy the idea of fish-do in the sink....
In the clean bucket you put cold water, with a little boiling water from the kettle to bring it nearer to tank temperature (I find 1 litre boiling to 9 ltrs cold is about right), add dechlorinator, swirl it round and then add gently to tank. (I use a plastic measurement jug- again for aquarium use only- because I find I can't pour from a bucket without upsetting everything. Also a handy piece of equipment for adding medicine etc). Dechlorinator should work pretty instantly.