Just briefly skimmed through this thread. There are a few things I think that need addressing.
Nothing you can do in this hobby will lead to immediate improvements. You need patience. Your tank is honestly still pretty new. From here, please pick a course of action and stick through it for like 3 or 4 weeks at least and see if things improve. If they do not, only
then do you change something.
The WRGB2 Slim can get pretty bright. I agree with
@FrozenShivers, I also would not consider your settings to be low. I'm using the same light as you (on a much smaller tank) and I don't believe my settings are as bright as yours but I am growing plants that are substantially more demanding. Based on the most recent photo of your tank there are hardly any plants in there that require such high levels of lighting. The plants in your tank appear to be Eleocharis sp, Hydrocotyle tripartita, bits of moss, Anubias, Bucephalandra, Java fern and like, one or two Rotala rotundifolia. Considering that your tank is still very new, you will face issues throwing that much light on a sparsely planted tank where the plant mass is almost exclusively slow growers.
Your drop checker is blue. Further, it’s blue even though you’ve appeared to have placed it directly above your CO2 diffuser. If your drop checker is still blue when CO2 bubbles are literally rising into the drop checker itself then there is either something wrong with the fluid in your drop checker, or you are barely dosing any CO2. In any case, drop checkers are not an incredibly accurate way to measure CO2 concentration. Read this,
especially point 3
How to measure CO2 levels in an aquarium
You need to carry out your maintenance properly. Clean the glass regularly and don’t just change water, but siphon any bits of algae and detritus you can get along with the water. Wave your hands about in the water and siphon any bits that fly up. I also saw one of your earlier posts where you shared your concerns about cherry shrimp being sensitive to water changes. I would suggest that you do at least 50% water change once a week
anyway. I regularly do at least 70% water changes and the shrimp don’t seem to care.
It is true that plants need adequate nutrition to thrive but I will have to respectfully disagree with
@hypnogogia that OP absolutely needs to add nitrates and phosphates to his tank through fertilisers. I believe 2hr Aquarist advises using APT1 for tanks with little plant mass and relatively high fish loads. That said, you will not experience any downside in using APT3 instead of 1 and will probably find it beneficial instead.
All the advice that has been given already is sound. I would suggest:
1. Cleaning the glass and manually removing as much algae as you possibly can, followed by a big water change whilst removing as much detritus and waste as possible.
2. Reducing brightness by maybe a third
3. Starting CO2 two hours before your lights turn on and one hour before your lights turn off, if you aren't already doing so.
4. Measuring your CO2 in the way described in the article, which is to aim for a drop of 1 pH before CO2 starts and shortly after your lights turn on
5. Dose APT
3 as directed on the bottle
6. Feeding your fish very small amounts and only once a day
7. Stop buying things that the guy at the fish shop says you need because chances are, you don't need those things (especially chemicals!)
8.
Regular weekly maintenance
