Just had an interesting observation the other day (by accident).

I know people have said that using hob's will create lots of co2 offgassing. Having hobs on all my tanks, I've never really known just how much of an effect they have. Well two days ago, there was a power outage. It wasn't long, but was enough for my hob in my 30g co2 tank to run dry. When the power came back on, it couldn't self prime so stayed inoperative for two days until I realized it. During those two days, I was amazed at how yellow my drop checker was. In fact, I was getting ready to change the fluid thinking it must have gotten contaminated (since I've NEVER gotten it above yellowish green before) when I realized that the hob wasn't working. So there you go. I did turn it back on, but this time on low flow. There was no ammonia readings at all, so I think ultimately I'm going to remove it altogether and just install a powerhead inside the tank for circulation (I'm going to set up a new 20g non-co2 tank so I'll use it on that instead). I do have an internal filter which just has a bit of course sponge on it and doubles as my co2 diffuser. So I wonder if we really need filters at all on planted tanks, or if just powerheads would be enough or maybe even better. After learning about saltwater tanks and how they have found that things work better if you just rely on circulation, live rock, and macroalgae for filtration, why are freshwater tanks so stuck on filters? Perhaps this is the way of the future.

thanks for the info, i had a similar epiphany when i found out my spray bar had a leak around the connector and was squirting and breaking the water surface. The co2 shift was not as dramatic but did take a lot longer to stabilize.

I recently stopped using HOB filters on co2 injected tanks. I also see significant outgassing. Also. I think HOB filters do not provide a very good circulation pattern.

On small tanks I now use the Magnum HOT filter.

It is possible to control co2 loss by being very careful to keep the water level up, but I decided it was not worth the effort.

Just checked my drop checker, and it's back to green today. Plus now I have bba. I'm going to get rid of that hob, but I want to keep it cycled for my new tank so I can't yet.