hydrogen peroxide in aquarium

Question
hi there.  i have been a water keeper (didn't get round to choosing the type of life to keep in my freshwater aquarium, 20Liters) for 5 months.  the tank is about 30% planted, and lit with 15watts plant flourescent 8 hours.  my regime re water changes has been sporadic to say the least, but only because i have only had plants and water.  there is a power filter with venturi, and the heat is 26C. well, everything has been fairly quiet in there for about 3 months, plants growing, one plant slowly disintergrating, a while a go some black algae nearly smothered a plant but i scrubbed it out, then a hair algae followed that... that brings the history up to today.  i decided i'd had enough of the hair algae, some of which  had started to die, so in went the tooth brush. there was a whitey proteiny like substance on the glass, which ended up everywhere. got some out though.  in my infinite wisdom (read stupidity), i decided to drop one drop of food grade H2O2 into the water (35%), and then thought it looked like a drop in the ocean. so in went another 2, no more than that. i thought this might kill the last of the algae, and as there were no fish or invertabrates, what could go wrong?  well, half an hour later the tank was stinking the room out. it was like eggs, and it got stronger and stronger. mostly noticable right above the venturi. anyway, tested amm, nit, nit and all fine, but pH was at 5. i assume i've crashed the pH, which would have killed any fish, but i'm sure i've killed the filter.  i've put half a teaspoon of bicarb in the water and the smell has subsided.  will check the pH later.  maybe you could tell me what you think i might have done adding the peroxide, in case i'm off, or part-right.

Answer
martin,
part of the problem with  H2O2  as as algae destroyer or tank sterilizer is that it is hard to get the exact measurement that you need as there are 100's of different opinions as to how much how often, what strength, etc etc.
Also, as an alagae killer it will also raise problems for the plants in the tank if you have them, as the cells of algae and plants are similar and the  H2O2  is not limited to nusaince algaes but will damage everything that is sensitive to it.
It will effectivly sterilize your tank like bleach will.
i would guess that you killed off some of your biofilter.
I would do a couple of 25% water changes at this point and try some practice runs on a test tank to get a level that will work for you without causing more trouble than it solves.
If you need further assistance, feel free to ask.