Ok... I just bought some 35% food grade h202... Also I asked the hydro store guy about the difference between that and the store bought 3%. His opinion was that they were two different types of h202. Personally I have had to do a lot of fighting with the 3% in high quantities. I use about 5 32oz thing of 3% h202 per week. That is about $1.50 per 32oz (about $8.00 a week), while I just bought the 35% flavour and it should last for about 3 weeks and only cost $9.00.
Also I have noticed a lot more bubbles and "Algae fighting" from the equivalent use of 35% then 3%. I would add like 12oz twice a day of 3% to my 23 gallon tubs, and now I only have to add about 1oz of 35%, and it seems like it is working a lot better..
Just a quick learning experience... Don't f*ck around with 35% h202. It likes to burn skin in its concentrate form. Turns like a white paint color. Very fun stuff...
Anyways... My vote is stay away from 3% because it is a pain in the arse to work with and for me cost a lot more in the end.
cheers