dark blotchy plants

[Coyne]

Some of my plants have developed dark blotches on their leaves. Im certain this isnt good, but am not sure what to do to either fix it or prevent it from happening again.
I have a low-tech, moderately planted tank. No CO2, but I do add the liquid carbon every day or two. I have the Seachem Flourish root tabs in the gravel and add Seachem Flourish liquid fertilizer twice a week. I have single 5000K "Flora Sun" bulb thats supposed to be designed for use with plants and it is on for 9 hours a day.
Any ideas?
Heres a pic:

 

[Rivercats]

Looks like phosphate deficiency to me. Do you know your nitrate and phosphate readings?

Also looking at how many of the leaves also have dark veins and lighter leaf tissue you might also have a magnesium deficiency issue too. Do you happen to know your tanks Gh?

 

[Coyne]

Ive never tested phosphates, but my nitrates were at 10ppm earlier today, but I did a 25% PWC as my nitrites were at .25ppm.
I have a hard time understanding my GH readings. Using the API test for it, it takes 8 drops for the color change.... so 143.2 ppm GH/KH? PH is a high 8.2 - 8.4 (in case that comes up later).

 

[Rivercats]

Just make sure your nitrates don't drop below 10ppm. You might have to get some Seachem Phosphate to dose your tank with. But first you can try increasing the amount of the fert your using. If you have some new leaf growth along with the bad leaves I'd cut the bad leaves off. Then increase your fert and see if the new leaves stay green. If not then you might have to invest in the other fert.

 

[Coyne]

After getting my hand in there to take a look at the other leaves I discovered that the vast majority were showing the same signs to some degree or another. Also, there were clumps of what I would assume is decaying material under both plants. I ended up removing both plants

Thanks for the info though. I'll have to invest in a phosphate tesk kit.
I was unaware that I wanted to keep nitrates above 10ppm for plants, so I'll have to keep that in mind as well when Im on the fence about doing a PWC.
I'll keep a closer eye on my other plants and see see if they start doing the same thing.

 

[Brian_Nano12g]

From the picture it looks like it's only your java fern going through this browning. Are there any other plants showing similar symptoms?

 

[Coyne]

no... I have some yellowing on some plants, but after posting about that I found out that they arent aquatic plants and are slowly drowning. That said, ever since I added the root tabs and changed the lights the yellowing hasnt progressed. I still plan on replacing them fairly soon though.

 

[Brian_Nano12g]

Okay i was going to say i had something similar happen to my java fern and it had nothing to do with nutrients. Look up "java fern disease." Anyway.. Good luck to your changes

 

[Coyne]

Im going to look it up anyways in case I decide to put more in. Thanks for heads up!

 

[Catfish Lover]

Also seems this could be a case of over dosing fertilizer. You said you're only running one bulb,if this is the case only one dose per week would be necessary.