I notice your near a window there. I often wonder if fluctuations in the co2 may be caused by the photo period :? For example my tank is next to a patio door which gets light early morning. On the sunniest days this light is quite intense but my lights on period is from 5 until 11 pm for optimum viewing time for me. I'm wondering if maybe light intensity and duration going up and down depending on the weather may be using loads co2 some days and some days not making the co2 requirement fluctuate.
The problem with the algae eating fish is getting hold of the right ones. Most LFS label them wrongly and not a lot I have found are the genuine species. Having said that keeping co2 stable, dosing EC everyday on to affected areas and trimming plants seems to be getting there. It has took months rather than weeks and I find thinking you can blitz it in a week with big doses of this and that and loads co2, lights out etc actually makes the situation worse.
You need to play the long(steady) game IMO.
