Definite CO2 distribution issue then.
Excel does help plants, not a 100% substitute for CO2 as far "weaker". Also does melt some plants even in small doses.
Does however kill BBA and some BGA. Either spot dose from a squirter/syringe onto BBA or dip affected plants in a diluted solution. If solution is too strong or dipped for too long will kill the leaves, bit of trial and error unfortunately. Or single dose at x5 (or x10) to kill all BBA in one go. Does work, but again can kill plants and fish...so be careful.
Have dipped all 4 stone inside excel solution diluted with 1:10 tap water. Then rinse it and drop back in. Will be grabbing the fertiliser and drop checker. Test kit gonna wait a little as it only arrive next week.
Lighting to 4 hour now too. and co2 to come on 2 hour before lighting and switch off 1 hour before lighting end @ 1.5bps