Chaeto losing it's real dark green color..

Heyy, I just had a question.. My chaeto is losing some of the deep green color, its a lighter green than it was.. My nitrates and phosphates are 0 and I use Phosguard...

Could there not be enough nutrients in the water for it to thrive? My tank is super clean and the nitrates and phosphates have read zero for a while now... what do you think? Should I stop using phosguard?