I've had a 120 Gal go well, then get a little algae off and on for a few months. Ever since I moved.
I knew it was CO2, the other tanks have all done well, but not this one.
I switched out some of the CO2 items, the reg and solenoid. Then added a good pair of ideal needle valves. Still issues here and there. I think I'd be very tempted to think it was nutrients, or maybe magic.......but I know that issue well and also know my other 4 tanks have the same nutrients/water changes etc.
I'd had the CO2 being fed into the reactor below, so I could not see it coming on/off much, just what was in the tank itself via plant growth/algae. I'd look here and there, but figured it was all new stuff, could not the problem.
Tank did better when I'd leave for a week etc, and it went limiting.........
What was the issue? Damn solenoid would stick open and close a few hours before switch on or off.
I spotted it last night, the bubbles still where on when I look under the tank at the reactor. I checked AM, and it was off, but then came on after 2-3 hours after the lights where on.
I pulled it out, replaced the solenoid, it's working right now.
It's also easier to gas fish when this happens.
I'm not inexperienced with issues about CO2, but it gets me every now and then too.
And if gets me, and gets the Amano, and anyone/everyone else a few times, do not feel bad if you think it's something else, but it turns out to be CO2 after all.
Importantly, do not give up looking for the issue. Some do and then claim nutrients are "the key". Nutrients can be ruled out and with other reference tanks (which I have, but some do not, nor enough experience to tell one way or the other) as well as light(I have a PAR meter and can adjust each tank to be identical by height adjustment). Certain species of algae are also indicative, and they can be added to other tanks and not persist like they did in this tank.
How long should a person look at CO2 and adjust, before giving up and trying something else?
Well, if the other options, lower light, sediment ferts, good water changes/dosing, clean filters, good high current, and they still cannot get a decent CO2...........I think those folks are rare.
But there are some that cannot, then they can go to the PO4 limiting method, but..........with the knowledge that CO2 is still a monkey on their back, they chose not to address. We can fix many things indirectly, but this does not address the root. Ultimately, most can, and this provides the higher level of wiggle room and better growth than limiting nutrients. But a few will never get some of these things right and it can be hard to resolve over the web.
I would had never caught about 20% of the issues I've seen in person for example about CO2.