As for iodine it helps significantly with Xenia growth - I dose, it grows. I don't dose - it still grows, just not the rapid-duplicating-monster.
If you can test for it - as Steve said above, total iodine levels - add a half-capful and test it in two hours. Keep it at that level. But then again -- 30g tank... I'd be too scared to dose anything with that little water. :| Unless you're interested in Xenia growth, or you just dont want your test kit to go to waste (
) -- dont dose iodine. :P