The diatoms are back

Diatoms can crop up in older tanks. I recently rescaped one of my tanks and removed a large amount of plants, and sure enough I got brown algae. Their growth stopped when the plants began to fill in.

To the OP, how much light are you using? I suspect it might be CO2 related.
 
Sorry--very busy at work today, only now having time to respond. The tank's been up and running since April 7, so it's 6 months old; fully cycled (0 ammonia, 0 nitrite, 10 ppm nitrate) since June 11. It's a glass tank. The diatoms appeared about a month into the initial cycle (mid-May) and were completely wiped out around end of July by what Red and I now fondly refer to as Rent-A-Pleco (Red really really wanted it so I relented but after I realized he'd gotten a sailfin, I insisted we take it back a week later).

I converted the tank from white Estes gravel & plastic plants to Eco-Complete & live plants on September 2 at which time I upgraded the lights from .55 WPG to 1.75 wpg, so the T-5s are about a month old. I retained all the filter media and there was no appreciable mini-cycle that I could discern after The Big Switch--ammonia and nitrite remained at zero. The diatoms reappeared end of September and are now everywhere again, as they were prior to Rent-A-Pleco. The platies and gouramis do love to peck at the diatoms but they can't do the scouring job Rent-A-Pleco did. I feed them in the morning and I don't feed them more than they can eat in 30-45 seconds. Red swears he only gives them an itty-bitty pinch in the afternoon (if he can't give them their "snack" it will make Red very sad). I do dose Excel daily but have no pressurized CO2.

I know the diatoms aren't harmful per se but I can't stand they way they look and my pennywort and bacopa is pretty well inundated with the stuff to the point that I wonder if the diatoms are letting enough light through for optimal growth!
 
Lighting isn't too high unless those T5s are high output.

I'd start dosing traces and bring the phosphate to 1, no point in limiting the plant growth and they don't cause algae if overdosed.
 
They are indeed HO T-5s. I have two of the 24" freshwater version of these lights over the 55 gallon:
http://www.drsfostersmith.com/Produ...ll&Ntx=mode+matchallpartial&Np=1&N=2004&Nty=1
I was warned they might be too high output for a non-CO2 tank. But, I thought diatoms preferred lower light, not being a true algae--is that not correct? (I've read conflicting things about diatoms and light so I'm not certain). I do have a little green algae, but that doesn't bug me--I kinda like its mossy look--bit I can't stand the dirty rusty diatoms. At least it's not as visible on the Eco-Complete as it was on the white gravel.

Our tap water has a silica content of 8.2 ppm. I don't know if that's high or low. I do think a big part of the problem is that except for the hornwort, none of the plants are really thriving. If the plant growth would take off, there would be fewer nutrients left for the diatoms. There is some new growth here and there, but nothing spectacular. It's hard for me to believe I have too much light in that regard--I expected the H. callitrichoides might not make it but even the ambulia (L. indica) and green temple plant (H. corymbosa) died.

Mr. and Mrs. Apple Snail were not responding well to one of the fertilizers, so I moved them to the 5 gallon and ramped up the Excel, NO3 and KCl dosing in the 55 gallon. I guess I'll start traces, bring up the phosphate ever so slightly and see if I can't get the plants growing mo' bettah.
 
Diatoms photosynthesize so they do like light, but they are able to tolerate lower light levels.

HC actually doesn't require high light, though it's a slow grower so it will look like it's not growing in lower light. It needs good CO2 though, haven't been successful growing HC without CO2.

Anyways, the problem isn't too much lighting alone, it's not enough CO2 and nutrients to go along with the higher light. Higher light drives faster growth, which requires more nutrients. Once a nutrient becomes deficient growth becomes erratic and stalls. Same thing can happen in a low light setup, don't do a water change for a couple of weeks and hello algae (unless you dose ferts).

So introduce the full range of nutrients to make sure nothing is deficient. Excel everyday can't be cheap, might be time to save up for a CO2 system.
 
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