nutrient ratios/fishfood bioload/terrestrial plant filters

[Dirk_Matthys at toyota-europe.com]

Hi everybody,

Please bare with me , it takes a while to read but I hope the triggered
discussion  can give more insight to nutrient balancing for me(all).

Background and set-up:
I currently have a well growing medium tech  75 G planted tank ( CO2,
2,5Watts/G of T8, substr heater, sump )  A few weeks ago my wife was
telling me how empty the wall was behind the tank. So she wanted a
philodendron to grow on the wall behind the tank. As expected my aquarium
oriented mind went to work right away and after having read about plant
filters I decided to feed the philodendron through my sump circuit. So I
modified a terrestrial plant reservoir with a drip feed and a drainage to
my sump. I filled the resevoir with "argex"( I think argex is the TM in
Europe ) . Basically they are the fired brown clay balls used in
hydroculture. I stuffed in about 12 shoots of a philodendron, made a raster
on the wall so the climbing shoots have a grip, rerouted some piping  Et
voila: Wife happy and Dirk has a plant filter on his tank. ( we'll see how
it works out in a month or two )
I have a medium fish load in this tank  , and I feed once or twice a day ,
mainly mosquito larvae ( all colours ) and live daphnia + some dry
commercial  flakes ( tetra) . Further fertilisation is only K2SO4 and TMG,
and rarely some KNO3. The nitrates in the tap have 7 ppm Nitrates so my
every 14 days  30% water change replenishes the nitrates on top of what
comes in as fish food.  Without the plant filter the tank was running
smooth , except when I stopped feeding the fish on a holiday or so, the
plants immediately reduced growth rate!

Question/issue:
There is plenty of information out there about the redfield ratio. Higher
N: P values promote higher plants and green algae, lower N:P favours
especially blue-greens ,16 to 1 is indicated somewhere as a good benchmark
for our aquariums and most plants. I took some measurements and as well
followed the "estimative index" approach described by Tom Barr ( is his
name in every mail ? )  . I assume that, in my "almost  no algae problems "
situation, using  above described inputs, I must be relatively close to
good N:P values. And  now I have introduced a terrestrial plant filter. My
question is how this may impact the nurient ratios.
What kind of nutrients does a terrestrial plant filter use?  Do terrestrial
plants consume N and P in similar ratios to aquatic plants?  Are
terrestrial plants better at consuming P than N or oppposite ?  far better?
do terrestrials use atmospheric N instead of NO3? Now the plant filter is
there,  I hope to be able to increase my fish bio-mass without compromising
too much on algae issues. If the plant filter only takes the N out of the
water I am facing a problem and will need to dose a lot of N otherwise  P
will build up , if a plant filter takes a good N:P ratio out of the water I
may be OK. If the plant filter takes mainly P, I am a happy man since al
lot of the  the P of fish foods would be absorbed easily.
If N and P are dosed only via fish food and water changes, (all other
nutrients are supplied chemically):  Does fish food give a reasonable N: P
balance?  or can I assume fish food to add too much of one . ( daphnia and
mosquito larvae and dry flakes, later I want to be able to use  beefheart
recipes ) Since I had reduced planth growth when reducing feeding I expect
this plant filter may compete too much with my aquarium plants . So is
there a risk of over filtering by using terrestrial plants? or would a
terrestrial plant filter throw off the balance by filtering out too much of
N or P that is introduced by increased feeding?

I have put quite some fish in this tank for one reason only: I want a
planted discus tank eventually ( Have experience with discus and plants ) I
figured that if I load that tank with plenty of fish and I can manage it
without algae, I can assume the same biomass in discus can replace the
current stocking level safely without algae.  I want this heavily planted +
philodendron filter tank  to support a family of 4-5 or may be 6 adult
discus + some small algae eating goodies .Do I stand a chance?

I know I have to base myself on meaurements later on , but I would
appreciate any input from people that have experience with plant filters
and higher bioloads then usually advocated for planted tanks. ( since plant
filters extend the possible uptake of nutrients which is nomally limited by
the amount of fast growing plants you can squeeze into your tank volume. )

( current fish in the 75 G : 15 full grown congo tetras, pair of kribs, 15
red phantoms, 8 rummynose tetras, 5 full grown SAE , some AFF and some
otto's, some Yamato-ebi japanese swamp shrimps )

thanks a lot

suisoman Dirk