This I've been doing since you recommended it in the fishkeepers thread. I dose flourish maybe once a fortnight and have been taking 4 litres out of 32 for a water change weekly. Lights are on 5-6 hours and the tank is very dim at other times, there is very little natural light in the room. I haven't seen any abatement in the brown wooly thread algae, but I notice that it only appears on the ambulia and the oldest strands of moss and dwarf chain sword. The crypts, val and lotus all have root fertilisers but I gather the other 3 must take their nutrients from the water column and maybe aren't getting enough so are therefore weaker so get the algae?? It hasn't abated but it also hasn't gotten worse.
Water parameters for ammonia, nitrite and nitrate seem the same, maybe slightly less nitrate. I will get the phospates tested next. I'm not keen to increase the lighting until I've seen how this low tech way pans out over a longer term.
I think it's been long enough that I can change something else now, do you think less water change amount, ie 2 litres, or 4 litres only every fortnight?
And another question - where do you measure the colour of the test tube? In one book it said to look straight down the tube from top to bottom, other info suggests comparing the colour of the tube long ways to the colour on the card is right. If it's the latter then the nitrate has never been over 10ppm, the other way is slightly more intense red.