Ah, I see, sorry.
Those changes were made over a period of over 6 months, one at a time with about a month in between each, some much longer. I don't know the exact timeline but I stopped using anti-algae meds and ferts before I switched to 50% RO in December last year. I tried adding the poly-filter media to the filter in early March if I remember right. I lost the betta about 4 weeks ago. Between December and now, the plants were mostly doing fine. I had to throw out some java moss because the hair algae had completely entangled it but other than that, the rest were surviving. I was doing 1/3 water changes every fortnight. I still tested ammonia, nitrite and nitrates before the water changes but was never able to read anything. It's entirely possible that the tank never cycled but I did add in and read ammonia during the attempt to fishlessly cycle, which was gone with no trace on further readings. I've been told that having live plants could potentially explain this but maybe that's not the case. I'm positive that the tank was not high in ammonia at any point while the betta was in it, though. Is there something else that the bacteria should be breaking down that might explain the condition of the tank if the cultures never grew?
EDIT: Substrate is gravel that was recommended for shrimp by the LFS. I'm fairly certain it was from Dennerle's Shrimp King range but I didn't take a note of it and don't have the packaging.
Ammonia cannot be broken down without nitrites. If you never got a reading of those, that means you wouldn't get a nitrate reading. Nitrogen cycle is: Ammonia ---> Nitrites ----> Nitrates. Nitrates are the least toxic of the three as long as the number doesn't get too high. Yes, plants can help with ammonia and remove it, but it isn't likely you had that many plants that were growing that fast enough to make that much of a difference. If they were growing that fast, your algae issue would have been minimal, if at all.
Fishless cycling requires patience. For some people it goes faster than others. It all depends on the makeup of the water being used.
The hair algae thing is annoying, but all new tanks break with some type of algae or many in some cases. Yes, it is ugly, but it can be dealt with fairly easily once the tank is stable biologically. I had to look up the substrate you're using, or believe you are using. Was it this one? https://www.aquaristikshop.com/aquar...e-Soil/907065/
I'm confused by it. It says it reduces hardness, but adds 'trace elements', whatever that means. Shrimp do not need special substrate. They like clean water and things to graze on.
What does your R/O water test at for TDS? Do you buy it or do you have a R/O system? If you have your own, how often do you change filters?
If your TDS is stable from the tap with R/O mixed in and you are getting these increases in TDS in the tank, I'd say it is the substrate putting stuff back in the water. This is simple enough to figure out with some experimenting.
Take the water mix you have been using ( tap water plus R/O) and put it in a bucket. Let it sit for 24 hours and test it. Do the same thing with some of the substrate added to a different bucket. Let it sit for 24 hours and test.