DIY CO2 + high is a very bad combo.
A good filter, good routine cleaning of the filter, will help to some degree.
But with high light/nuttrients, then you stop adding CO2 for a bit, that also induced GW in several tanks I tested.
Less CO2=> less N uptake.
This may result in more NH4 loading (this NH4 load remains unchanged(the rate of loading into the tank). But the rate of NH4 uptake is regulated by ther carbon supply to some degree, thus is downregulated by the plant's uptake enzymes when less CO2 is available and in a limiting amount.
That's the simplified version.
The molecular version is much more involved.
The potential toxicity of elevated ambient concentrations of NH4+ severely limits the range of concentration over which adaptation is possible. NO3, this is not the case, it can possess a much wider range of concentrations.
NH4+ disspates transmembrane proton gradients that are needed for photosynthesis and respiration electron transport.
The plant has much less leeway here with respect to NH4+.
What regulates the acqusition of NH4 with respect to CO2?
What carbon backbone is required to make glutamate? The precursor to glutamine? NH4+ is the precursor to Glutamine.
If the TCA cycle, the cycle supplying the carbon to the plant, is reduce by reducing the CO2 supply.............anything that disrupts the production of Glutamate or it's precursors will secondarily disrupt the assimilation of NH4, which in turn tell the NH4 uptake enzyme to stop taking in NH4 since there are less Carbons than are needed to forum glutamine.
So if you have a steady rate of NH4 being removed via plants/bacteria, then you do something under high light/nutrients to disrupt the bacteria/plant's uptake, then you have a back up and elevated levels of NH4.
Note, a small change in NH4+ levels in the water column causes a much more sensitive response than NO3 might.
I shut off my filter to an otherwise well run tank, high light, EI etc. And added a similar flow pattern and flow rate for the tank with a powerhead, I got greenwater within 30 hours.
I just did this 3 days ago.
I tried this with lower CO2 some years back, this produced a relapse of GW blooms after a few days past the the last UV sterilizing treatment.
Adding enough CO2 did not induce the GW bloom back again thereafter, but the sub optimal level of CO2 did induce a bloom.
You can try these ideas out and see if you can find similar experiences.
You can also add inorganic NH4, or urea, or organic forms of reduced N and see if you get GW(you should) at fairly small ranges.
Then kill the GW with the UV and try it again.
Note: such test tend to only hurt the Algae, the plants typically do fine/very well.