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Fish load .v. plant load

Just out of interest...

In my 55gal (UK) over-stocked, moderately-planted tank I have never managed
to get to the position of being Nitrate limited.

I currently have about 1.5W/gal of light & no CO2 & I suspect that one or
other (or both) of these are currently the limiting factor regarding plant
growth (as I use PMDD, my plants look good & I have to remove a modest
amount, mainly Vallis, from the tank every two weeks or so).

What I would like to understand is whether, if I were to increase my light &
add CO2, I could ever reasonably get to the point where the plants eat all
of the ammonia produced by the fish?

Has anyone ever done a rough quantitative analysis of this? i.e. the
relationship between "fish-food-in (grams/month), plant-material-removed
(grams/month) & rate-of-Nitrate-change (ppm/month) given a particular water
volume & static fish mass increase (i.e. Nitrogen-into-fish =

My fish food is 48% protein, & 100g lasts me about 4 months - does anyone
know how much Nitrogen that represents?  What is the Nitrogen content of
'wet Vallis'?

When plants uptake Nitrogen do they preferentially take Ammonia rather than
Nitrate or do they take both simultaneously if both are present?

Also, do, fish faeces biodegrade into something other than Ammonia (related
to the previous point)?