Anubia red spots

[Eti]

Hi,

I have a 55G aquarium (24 inches deep, 30 inches long, 15 wide).
I dose some excel, have 4 x 24wT5Ho lamps, eco-complete fluourite and liquid Seachem plant fertilizer.
I leave the light on for 12 hours a day.
I have noticed these red/brown spots on my anubias, do you know where it comes from? Too much light, not enough CO2 (I put excel everyother day)
I am really considering buying a CO2 diffuser, do you have any brand to recommend? Something simple to install and efficient?
Thanks for your help!
You can check the picture to understand what is going on with my plant, thanks!

[discus daddy]

though anubias is one of the hardiest plant in aquarium,but to see its original beauty and normal growth one has to keep them under some shading.so do you use any shading plants? also IMO your photo period is too long.you can use some siesta period while dealing with plants.

you have approx 46 gallon tank and you use 96W light.so light is not too high in this case.you may use some fern or crypt sp to hide anubias from direct light.this brown or yellow spot and sometimes you'll find black spot also due to direct light.