based on a search most of finlands drinking water comes from surface water . Surface water is mainly from rain or snow melt. Your low GH is consistent with mostly rain water in your tap. It doesn't mater if the water is treated with chlorine or chloramine. What maters is how much chlorine is and sulfur is in your water. Cl and S are necessary plant nutrients. Plants cannot grow without them. If you don't have enough plant growth will slow or stop while algae keeps growing.
The reason I recommended a GH booster was to insure enough S and to a lesser extent enough Mg. The GH booster you looked at doesn't say if it has any S or Cl in it. According to the web site. It just list N, Mg, Ca. Yes with it you can reduce K but your K is not high enough to worry about. What is more important it to insure your don't run out of of the nutrients most people never talk about.
I would have to disagree with this. MY tank had a lot of algae issues for years. I following the common advise to reduce lighting and keep the tank clean with large water change. I still had algae and little to no plant growth. I took me years to identify all of the nutrients deficiencies that were causing most of my issues. Was K ever a problem in my tank No, but N, iron, Mg, Ca, S, Cl, Mn, Cu and possibly zinc were. I never found one fertlizler that would solve all of the problems I was finding. Most on the market will work with medium harness tap water but I was using RO water with an inert substrate. Once I made my own GH and micro fertilizer algae growth was greatly reduce or stopped. I am not at zero algae but I am closer than I ever have been.
In my medium light tank I am dosing:
3 times a week
Once per week
and then use a MgS04, CaCl2 for Ca, Mg, S, Cl GH booster (3 parts Ca to 1 Mg) to keep my Gh stable.
Their recommendation has a lot more of many nutrients, Has no Cl and S and and in my opinion extremely low Ca and Mg, and way more micros than your plants can use in a week. With a high light tank nutrient consumption would be higher but still not enough to consume all the micros. Their recommendation makes no sense unless they are trying to make more money.
I feel a bit uneasy about purposefully dosing Chlorine into an aquarium with Caridina species shrimp. I know that Prime binds nitrites and ammonia leaving them bioavailable, but what do they do with Chlorine/Chloramine? Do they bind it leaving it up for plants to consume, or does it destroy it altogether?
Also I think growth has been good. Here you can see a GIF of the past 7 day progress
I have good growth but I also don't seem to be get rid of algae growth either.
I just did a comprehensive water test before changing water.
I dosed these amounts (total of 31ppm of nutrients, 56ml of ferts):
and this is what I measured - here is an IMGUR album with pictures of the test results https://imgur.com/a/Z1vMeWG
NO3: 5-10 mg / 25mg (latter is from "more accurate" kit)
PO4: off the scale (>1.8ppm)
K: off the scale (>15ppm)
pH 6.9 (degassed)
pH has increased quite a bit. I think this is related to me testing what happens to water temps if I keep the lid open. The CO2 that isn't dissolved gets out from the tank and is no longer in surface contact with the water as much -> CO2 concentration goes down -> pH goes up. TDS increased by 100. GH and KH increased by 2 degrees..
I guess those results are OK, but still it seems odd that adding 30ppm worth of nutrients increases the TDS by 100. Also seems odd that NO3 would be up to even 25mg/l range after dosing only 15 mg/l. I know fish poop adds some, but my plant mass is huge, the biggest it has ever been (with the exception of the echinodorus which I brutally trimmed to get rid of algae infested leaves). The ADA Aquasoil may be leeching some? Tank is on day 113.
Doing a 50% WC and measuring everything again. Also diluting 1ml of macros into a bucket and testing whether the concentrations are actually what they say they should be.
Post 50% WC numbers are as follows:
TDS 190 ppm
Fe 0.05 mg/l