When clorinated water is safe to put into your tank.And I have heard if left too long it becomes bad for your fish.What does adding an airstone do to the water.

the easiest thing to do is buy some Aquasafe or another product that makes the water safe for the scaled ones and then no waiting

apart from that I am not 100% sure how long it takes the chlorine gas to dissipate once it's out of the tap

Adding an airstone breaks the surface of the water and assists with oxygen exchange. If the water is chlorinated it would help to dissipate the chlorine a bit quicker than if it is just left sitting.

Another way you can do it is to fill a bucket from a garden sprinkler so that the water is well aerated as it goes into the bucket. I don't think I would trust it totally though - I would still use some Stress Coat or similar. When I want to freshen my ponds I just put the sprinkler on them for a while but that's only a partial water change and it's very well aerated.

In some countries monochloramine is used to disinfect water supplies rather than Chlorine. It is used because chlorine is a strong oxidizing agent and reacts with other compounds in the water supply to form nasties which monochloramine does not. It is used in about 25% of the USA.

It is a requirement of the Department of Health in NZ that chlorine is used as it is a better disinfectant than monochloramine. All proteins are a double helix of phosphates with various amino acids hanging off them and all living things contain proteins. Chlorine reacts with amino acids and other nitrogen compounds to form monochloramine, then dichloramine and then trichloramine. This is called the chlorine demand when chlorinating water supplies and there will be no free available chlorine until this demand is satisfied. As the chlorine dissapates the equilibrium changes and the chloramines move back to monochloramine and this is the compound that irritates your eyes in a swimming pool and the compound that people think they are smelling as chlorine. The way to fix that problem is to add more chlorine and push the chloramines up to the trichloramine state and this is not so irritating.

It is always the case therefore that chlorinated water supplies contain chloramines and they will not be removed by storage, heating or aeration, but will be moved to the monochloramine state which is the one that is most dangerous to your fish. The only way to remove the monochloramine is with chemicals such as sodium thiosulphate which is the active ingredient in treatments available from the pet shop. Removing the monochloramine this way will release some ammonia but it is generally in the more harmless ammonium state.

I change water straight from the tap and have done for 30 years and never had any problems as regarding chlorine. However it is not (imo) a good idea if you intend to breed. I think chlorine dissipates within 24 hrs.

It probably dissipates quicker than that. My point is that the chloramines don't.

so if I fill a wheely bin with tap water throw in a heater and airstone then draw from it over a week I could be hurting my fish.

Yes.

alan has explained it all, but if you are looking for a cheaper alternative to aquaplus and stress coat, try prime. it is more expensive to purchase but 5mL instantly neutralises nasties from 200L of water, so is far cheaper to use over the course of time.

cf. aqua+ and stress coat - 5mL neutralises 37.5L of water.

I use prime, as Phoenix said 5ml does the 200L barrel I use for water changes. That said I usually do 250-350L water changes and top the rest up straight from the tap with no hassles.