poor CO2 distribution to the carpet - Poor flow/turnover
got this plastic tub with lid on it in the Garden with off cuts of plants, no ferts just toss off cuts in it
The MC above the water looks great, the MC under the water is 'Yellow' whats the other difference CO2 above water lots, below water poor CO2 same ferts light etc etc reason its yellow not enough CO2 in water your same at carpet level
Tropica premium doesn't contain any nitrogen, this is probably ok for a low tech but you're using co2. Your substrate probably contained nitrogen initially but this has probably now been depleted to some extent, hence the deficienes starting to now show. Switch to tropica specialised which is a complete fertiliser containing nitrogen (the green one), should hopefully help clear up any deficiencies.
I once had the very same issue with hair gras with co².. Remarkable is, i have the very same plant in a rather low flow, low light and non co2 tank. There it grows 10 x slower but it never shows any deficiencies, it's always healthy.
I'm not betting because i dislike gambling.. But it likely is a combination of things and since co² speeds up a lot of processes related, the first best obvious guess any co² user would say it's a co² issue. I experience it a lot that if co² is used, it's the usual suspect if something is wrong. But i'm not so sure about that at all..
To narrow this down you first need to be absolutely sure you add sufficient ferts. Than after that you need to investigate if there is sufficient flow. My guess is, since this plant can grow healthy in a non co² invironment that little or not enough co² is the last suspect to worry about. Maybe it is fluctuating co² parameters driving the plants nuts, dunno if that effect rather causes algae growth or unhealthy plant growth. But if you add it, it rather should be stable.
The most frustrating thing is, once you have a lawn of hair grass showing this issue, there is very little you can do than trim it all back regorously. It is near impossible to clip out every single unhealthy leaf. Trimming it back as short as possible, get all debri out.. Leaving it as is with all the dead leaves melting in it will only cause extra bioload and makes it susceptible to algae growth on the dead leaves jumping over on the healthy ones.
Make sure the ferts and the flow is ok, make sure the co2 is stable. And wait for new healthy growth.. Me too i experienced this as very painfull and frustrating, cutting all back and having an unsightly front lawn for weeks.. But it did the trick.. In my case it was a fert issue, because i didn't change the co² and also not the flow. I changed to dosing dry salts..