H202 is poison! Physan 20 does the same and it breaks down into nitrogen that you're plants can use. aqua shield is watered down physan 20, it costs more too.Aquashield is a great product mix it into your res with a little bit of Sweet. And spray it on the roots, the pythium will be gone in a day. Some people prefer to use H202 or Hygrozyme, any will do.
It kills all the benificial microbes your plants need and thrive from. Cheap yes, but who says cheaper is better. Quality comes from reserch and experence. Physan 20 is made for plants and gardens, H202 is not. Physan 20 is stable and cannot build up over time or leave resedue. H202 will sterilize your root system and possibly last longer than necessary or expected.h2o2 works great. also super cheap and available.
i use it all the time. just use a little bit at a time, it just takes like a day or so to degrade into h2o and oxygen. so apply more in two days if problem isn't fixed.