I have a few bushes of ceratophyllum Demersum in my tank.
When they were put in they were dark - light green - some of the whorls on
some of the plants were dying as you would expect. Now however, they have
mostly gone a copper red colour - except for a few. I do not have strong
lighting and was told that these are low'ish light plants. They have become
a bit stragly but are still attractive.

I inject CO2, the chemistry for PO4 = 0.25, Nitrate, Nitrite Ammonia are all
0. I have started putting in some liquid fertiliser.
The substrate is pure Flourite. The lighting is 2 tritons and a flora glo
tube. Watts per gallon does not add up (it's fairly low light in that
respect about 1.33w/gall but I do have parabolic reflectors.

Ant idea why these plants are discolouring so badly?

They also accumulate detritus like there is no tomorrow despite the water
being very clean. Also the surface accumulates dust and ultra-fine detritus
which I think is stopping the light reaching the substrate. I have a Eheim
filter turning over 660l/hr for a 250l tank - I have just replaced the
venturi inlet with a spray bar (which is under the surface) and placed the
outlet in a more visible place. The outlet I believ is doing a very poor job
of collecting the dust and fine detritus as it floats up to the surface.

Again any suggestions are welcome.

Martin