"Alan Silver"
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...
Hello,

I have a 150 litre tank, with 3x40W tubes, DIY CO2 and PMDD, soft
slightly acidic water. Fish load is moderate and the plant list is :-


I often see masses of bubbles, so I know there's photosynthesis going
on.

I started dosing with a few drops of PMDD every other day, but increased
this when nothing was happening. I now pour the stuff in, far more than
everyone seems to recommend. Algae is almost zero, unless I don't do
anything to the tank for two weeks, in which case there's a light coat
of it on the glass which cleans off easily.

Any ideas ? TIA

In your other post, you mentioned that your nitrate is almost 0. With over 3
Watts per gallon, that's a dangerously low level to have. I'm surprised you
don't have much algae. If you're using the standard PMDD mixture, you should
add enough to maintain an NO3 level of around 10 ppm. The lack of
macronutrients N, K, and P would explain the slow growth. Make sure your NO3
test kit is accurate and keep adding until you get 10 ppm or more NO3. Don't
let it fall below 5 ppm.
__
Alex