Yellow Leaves With Brown Spots, Not Looking Good?

Hello.

I have a 125, planted Fluval Roma which has been fully cycled for months now and I have been experiencing no problems with anything up until now. In the last week or two, I have noticed that my plants are fading to a yellow tinge and now there are brown spots appearing on the leaves, as if the leaves are beginning to rot. This only seems to be true for the Alternathera and my Bacopa doesn't look too good, but the Alternathera is the worst. I have also planted some Eleocharis recently which doesn't seem to be growing especially well atm. Nitrates are low (5-15ppm) and everything else seems fine. My other plants are doing well, especially the Amazon sword which I have had for a while and the Crypt. I have been adding liquid fertiliser (only one dose a couple of days ago) for micronutrients, but I am worried that it may not be anything to do with nutrients and potenitally some kind of disease has hit the tank, considering the apparent species specific decay? Or is it simply a lack of something in the tank? I am especially worried that this will become a big issue very quickly......

Thanks!