Im probably a bit late here you probably have this done allready.
I would fertilise as normal but up the KNO3 element of the nitrates and not depend so much on the organic nitrates from the fish waste.
If it was me i would probably do a bigger water change to reduce the organic nitrates a little further, i would probably do 60% weekly water changes instead of 50% this will bring you down to about 10ppm nitrates and then i would add 10ppm KNO3 on top of this.
But i wouldnt worry about this if you have done the water change allready, i would just add 10ppm KNO3 regardless.
During the week i would add at least another 5ppm KNO3 to keep things topped up and if the uptake rate was higher add more KNO3, but i think that amount should be ok and will balance out the organic nitrate.
The important thing is to add the inorganic KNO3 as this reduces your chance of algae.
In low tech low light tanks the fishwaste and fish food is enough to provide the plants with the nutrients they require but in highlight tanks you need to top this up with inorganic nutrients.
A couple of quotes from Tom Barr the EI guru on this particular subject.
As you increase the lighting, you must also increase the CO2 and thus the nutrients in inorganic forms(non NH4 based fertilizers).
As you increase the turnover, lighting and growth rate of a planted tank, you must also reduce the amount of organic matter in the substrate(messy and has NH4+ high light = algae)
He was talking about the difference between low tech and high tech tanks as you increased the lighting you must also increase the nutrients in inorganic forms (KNO3,PO4 etc the stuff we add) and not to rely on NH4 based fertilisers ie. fish food or fish waste (the NH4 is ammonium, a by product of fish waste which can induce algae)
In the second quote (both from the same passage btw) he talks about trying to reduce the waste material in the substrate in order to reduce nitrates particularily, so a regular gravel vac would be a very good idea as well if you have a tank that is produceing high levels of organic waste which will in time turn into nitrates.
Anyway the point is not to rely on the fish waste alone, you must try and remove the extra nitrate by water changes and regular maintainence of the substrate, but thankfully your nitrate problem is very manageable with the above measures and alls you have to do is add KNO3 on a regular basis to top off what allready exists in the tank.
On the last point i would also wait until next sunday to do a water change i wouldnt be in any hurry to get rid of the maracyn to early in case it is of benifit and still working, unless of course it specifically tells you in the instructions to do so.
Anyway Sam i really hope this is it and the bga will be no more and your tank can return to normal, i will have my fingers crossed that it will work, im sure it will.
I hope the above makes a bit of sense, if you are not sure on anything just ask.................best of luck with it.
