this is probably one of the stupidest questions to ask but do feel it needs answering so please bare with me.... but here it goes, in order to bring down the pH to make the water more acidic in a fish tank and you dont want to use chemicals,why cant you use something like squeezed lemon juice,(or other naturally occuring acids,) to bring down the alkalinity?

Three basic reasons - firstly lemon juice will act in the same way as chemicals like pH Down, which are essentially just dilute acids and in exactly the same way will cause fluctuations in pH which are really bad for fish. Secondly, lemon juice will foul the water badly. Thirdly, lemon juice also contains fruit sugars which you don't want in the water as that would be harmful to fish and encourage growth of unwanted moulds, fungi and bacteria leading to infection.

so vinegar is a NO then only kidding.had to ask. basic chemistry was never my forte

operation pH down will be commencing soon though.just waiting for my bogwood,peat,leaves and fluval u1