Biofilm growth in h2o2'd water

is it dark? is it cool temps? If not, you are exacerbating any problem that is occurring. Regardless every couple weeks or a month you'll want to clean anything constantly holding water. i don't use anything in my 50-gal reservoir -- benes or sanitary products. Though it doesn't last more than 3-4 days and then at least a 75% refill with fresh water + nutes. The lowest it sits overnight is 50% full - i use about 25% per irrigation. This process keeps it biofilm free for 30+ days and no odor. This is when i empty and scrub it down for maintenance. Agitation is key too. I have a powerhead that keeps it circulating and oxygenated. Bubbles in a small bucket should be fine for this. A process where you refill a bit mroe often might help - not leaving water levels terribly low for a long time can help. Growth per volume is much greater at lower volumes and fouls faster. If refilling at 25% maybe try 50% and see how it goes. Might prolong the tme before you need a deep clean. temperature and light are killers for a rez or dwc bucket (just a tiny rez, essentially).

could use other nutrients and might fix it?

After 2 days all of your h2o2 degrades to water and oxygen. You want to refill every day to every second day. I most often give full dose when setting up going with 0.75-1ml with 12%hydrogenperoxide. Second day you can give half, 3rd day full dose again. And sometimes only every two days full dose. You need to play a little with the concentration, first you don't want to burn your roots and put oxidative Stress on your plants, second you want the bad bacteria in your nutrients reservoir to not recover from the desinfactant, in certain cases this can lead to bad bacteria becoming resistant to used H2o2 concentration.

H2O2 is really only effective for about 24 hours before it loses its extra oxygen and becomes plain water. To protect your rez from growing nasties you mainly need plenty of O2 and control your temps. They do make beneficial bacteria you can add too. I use a product called Hydroguard and my roots love it.