If your tank is new from May, then it's right on the 8 month mark where its common for the waste that was absorbed in your rock and sand to start leaching back out to your system.
You have an excess waste issue of both no3 and po4 that is not impacted but how much you are feeding now. Its the accumulation of how much you fed in the last 8 months when you have a young tank that is not mature and effective nutrient export yet. Your test results doesn't give you a full picture because remember it is only testing "whats left on the water". You no3/po4 levels may have tested fine in the past bc the excess waste was absorbed in the rock/sand. Now it tests fine because the excess waste is already absorbed by your GHA
This is a very common issue in young tanks - especially young small tanks. Only long term natural solution is time and manual removal. Time will mature your tank and increase the biodiversity and competition for nutrients that will keep gha in check.
How have your chaeto been growing? Keep in mind that just bc you have chaeto, doesn't meant it is absorbing all that excess waste. Remember it has to outcompete everything else in your DT for those waste - outcompete lighting, biomass, surface area, flow, etc...if your GHA already takes hold, the chaeto is not going to be effective. Ie, if you have 4x as much gha than chaeto, then 4/5 of your waste will be absorbed by gha instread. And this assumes that ur chaeto receives the same amount of light as gha, which often is not the case bc we have much fancier light in DT than in Fuge.
Upgrading your fuge light will help outcompete the gha as well
Gfo and vibrant could also work but I wouldn't recommend that in such a young tank bc it doesnt help with the long term maturation process of the tank.
