Try checking your water report. GH is basically the sum of Ca and Mg, without the report it is hard to assume something, but adding MgSO4/Epsom salts will not hurt. Adding 1-2 dGH from Seachem Equilibrium is also possible in case you can't find MgSO4, but you would also be increasing Ca.
Take a step back and consider the whole picture.
1. Lighting is the driver of growth, lots of light, lots of everything.
2. CO2 can be measured by the pH drop. 1 point pH drop is okay for most people. BPS is just a visual cue to you that things are working, not a measure.
Turning it on before lights is also a common practice.
3. Ferts. I cannot over emphasise this: know what you are dosing. Forget the brand, instructions, etc. They are useful, but what does it mean "5ml of this product in 50l?" As others pointed out, go for the concentrations, ppm, mg/l, etc. You have to provide 2 basic things: Macros (NPK, usually in the form of Nitrates, Phosphates and Potassium) and Micros (Fe, along with the commonly used EDTA or DTPA chelators, plus trace elements, like Zn, Mn, Cu, Mo, etc.).
4. Circulation is sometimes neglected. If water doesnt go around bringing all the good stuff (ferts and co2), then plants will have a problem. Same thing for Biomass, not only it block the water access, but it also creates competition. Light and CO2 competition is something not to be ignored 😉
5. Cleaniness. People forget to clean their filters (clean it once a month, attention to the bio-media), skip the water changes (bi-weekly at least, on a high tech tank), let too much mulm accumulate on the substrate (vacuum it), hoses completely filled with algae, etc. Keep biological waste down and things will get easier.
6. Patience. Introducing constant changes will not help. The mechanisms through which plants acquire nutrients, for instance the enzymes, take a while to adjust (read: weeks). Introduce changes, give it time (again, weeks).
One tank can improve significantly in 2 weeks, and grow wonders within 2 months of good care.
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