Jeyes "Kleen Off" Ammonia. Most household stores and DIY stores such as Robert Dyas etc. You are unlikely to find it in normal supermarkets though.
It's about £2 for a large bottle and more than you would likely use.
Edit: actually here is a good thread about it that will also show you a picture Kleen Off
Not used it and really don't think it's a good idea just use the right stuff in the right dose and you should be absolutely fine (or your water at least!)
If Boots don't do it check any local hardware store. As long as it's only ammonia and water (no perfume, etc), the brand doesn't matter. Mine cost 1.59 and I reackon there's enough in there to cycle a whole fish shop and leave some over for the spring clean.
It's not all that easy to find. Try Robert Dyas or a local DIY store. In the link I provided above it shows you exactly what it looks like. That's your safest bet as some of the others I've seen wasn't clear about what it contained and/or any additives. Where do you live. That stuff can't be posted, but if you wanted to have some, I'm close to Richmond (greater London).