I don't think that this is the reason for the surface scum etc. In a high productivity system everything is enhanced, the plants grow more quickly, but they are "leaky" structures, and are constantly losing sugars, lipids and proteins to the surrounding water. These substrates support the growth of bacteria, the more substrate you have the more bacteria, and I should imagine that many of these visible signs are a mixture of the substrates and their bacteria.Why it's not OK to have it yellow all day, I'm not a biologist but I'm guessing that if the PH & O2 stay low all day the bacterial activity is slowed down and you are getting an unprocessed organic matter build up that is further used by algae to develop (you can notice it very easy, surface scum, slimy glass etc.).
OK, that sounds like the carbonate reserve has been completely depleted and this means that there isn't a carbonate ~ CO2 equilibrium (details here: <http://en.wikipedia.org/wiki/Calcium_carbonate#Calcination_equilibrium>)......here are the water test results at the end of the photo period: KH - 0 (or close), can't test the ph properly, it's bellow tester's scale (5-10).
<http://www.ukaps.org/forum/viewtopic.php?f=51&t=18116&p=185726&hilit=+carbonate+dKH#p185726>Lets look at the opposite case. CO2 causes the water to be very acidic by lowering the pH of the water. We know that CO2 is a highly toxic substance. But what is the fundamental nature of the toxicity? is it because it causes the pH of the water column to fall? No. It's because CO2 in the water column prevents the fishes body from purging itself of the CO2 generated internally. Normally, CO2 travels from the blood, out to the water from across the gill membrane. High CO2 pressure within the water column prevents this exchange, so the CO2 inside the fishes bloodstream builds to high levels. that then lowers the pH inside the fish's blood. That causes all sorts of problems if their system is unable to control the internal drop in pH. The fish could not care less about the pH drop externally. It is the internal chemical reactions that causes problems and those internal problems are caused by the dissolved CO2 of the water, not the pH of the water.
cheers Darrel1.5g NaHCO3 in 25 litres of water = 2 dKH
1.8g KHCO3 in 25 litres of water = 2 dKH
1.2g K2CO3 in 25 litres of water = 2 dKH
Seems sensible, I would have expected that your fish would be all right, I'm pretty sure that pH is largely irrelevant to them (although not to fish from heavily buffered water).My fish are alright, most of them are soft water fish, they don't seem to be bothered.
The PH drop is more or less due to the RO water I use and acids from Amazonia & wood. I usually keep the drop checker green. I did some buffering yesterday, my aim is around 2-3 dKH not for the PH sake but to improve a bit the bacterial activity to break the waste that keeps building in my tank.
You could use always use sodium bicarbonate /carbonate (NaHCO3 / Na2CO3), they wouldn't offer the K+ source, but the they would have a similar effect on dKH.Hope I can find a cheap KHCO3 / K2CO3 source.