Odd new growth on Java Fern: holes?
I have a 65 gallon set-up that's pretty heavily planted. 36Lx24Hx18D tank with pressurized CO2 and 2x39W T5HO lighting. Mostly Crypts, Anubias, Java Fern, and a single large E. cordifolius. I also have a lot of Amazon Frogbit and floating Pennywort. Water parameters, as of 12/5/12, are:
GH = 6
KH = 9
pH = 7.0
Ammonia = 0ppm
Nitrite = 0ppm
Nitrate = 5ppm
I keep the CO2 levels around 25 to 30ppm, and there's not much algae to speak off, other than a bit of green dust and diatoms (enough to keep my Oto and Nerites fed). Tank stock is: 1 anglefish, 10 red-eye tetras, 14 cories, and 1 lonely Oto. All fish are very healthy.
Over the past month or so, new growth on my Java Ferns is growing in strangely. As the leaf grows, holes form occasionally in the new leaves. The holes tend to be not-uniform in shape or size, and tend to be in the upper half of the leaf. Sometimes, the tip of the leaf will end up slightly malformed. Smaller new growth tends to be more adversely affected, whereas larger new growth tends to grow more closely to normal. All other plants are very healthy with no noticeable deficiencies.
I dose as follows:
K2SO4 - 1/2 tsp. twice weekly
KNO3 - almost stopped dosing this entirely. Seems the fish are able to keep NO3 around 5 to 10ppm.
KH2PO4 - 1/8 tsp. twice weekly
CSM+B - 1/8 tsp. twice weekly
Fe chelate - 1/8 tsp. twice weekly
Water changes are now monthly, with water parameters checked weekly and so far have been very stable.
Any ideas what the problem could be with my Java Ferns?
Help is appreciated!