I read lots of people buy their scrubbies from dollar stores. You may also want to get a mesh bag too from the dollar store. I'm kinda paranoid when it comes to this stuff. I would rather go for proven scubbies vs unproven. Too bad, I want the unproven route but everything seems to be okay.
I bought mine from the dollar store. There were two versions. One was 10 in a bag and 12 in a big. Difference was, the 10 in a bag (upon careful examination) the dye was actually coloured bits that is stuck on clear fibers.
The 12 in 1 bag colours were dyed into the fibers and decided to buy 120 bags of those.
I soaked them a long time. Both did produce some bubbly substance. But I ended up with the ones dyed into the fibers for two reasons: 1) dyed bits would eventually flake off over time 2) it still had that 'new' smell after 3 soakings.
Almost 1 week in. Koi eating like pigs. Healthy as can be. Water crystal clear (I didn't add bacteria supplement). Then again, I have 16 4.5-7" koi... basically 1 koi per 100G.