Silicates, Diatoms, Si test kits

[Arthur1]

Where to start so I can keep this short... A 300 gallon fowlr aquarium that has given me an ongoig problem with diatoms is driving me a little nuts. I have a spectra pure max cap ro/di unit to help purify the incoming tap water. I just changed the membrane, di cartridges, carbon block all within the last 3 months and the sediment filter gets changed a little more often. Adjusted the waste water to product ratio etc. when I installed the new membrane. Phosphates are "zero" using a salifert test kit and usually .03 on the hanna hand held meter, nitrates undetectable as well. I do go through about 1.5lbs of ferric oxide a month on average. Not even 24 hours later there is a skim of diatoms of the surface of the sand. I've tested the aquarium with a seachem silcate test kit when I came to the conclusion that these diatoms weren't receding in the low range and it reads zero. I tested the tap today and according to the test kit it is at least 8mg/l, I tested the product water out of the ro/di unit with the low range method and I did get a reading of approximately 1mg/l and the directions say to then divide by 4 so, .25mg/l. The problem is I'm associating these diatoms to the silicates and one question is could the di resin from both cartridges be exhausted already? I talked to someone about it today, won't mention who, and this individual dismissed hobbyists test kits for silicates saying that they're not reliable and to not ever believe in their results. Someone will probably ask about a skimmer, its the old orca 250, can't wait for the upgrade Sorry to be all over the place but without writing an essay lol

 

[bertoni]

The problem might be cyanobacteria or dinoflagellates, as well, but that's not all that important. There might be some silicate in the output of the DI phase. There are special cartridges that bind more silicate, like the Silica Buster from Kent, that you could consider trying.

Are you saying that the bloom goes away when you add fresh GFO?

 

[downbeach]

What are you using for substrate?

 

[Arthur1]

Bertoni, I have been using spectrapures's cartridges the max cap followed by their silica buster cartridges, but if another route is better I would take it. No the diatoms do not seem to be affected much if at all no matter the volume of ferric oxide I use. I was thinking about "experimenting" with lanthanum chloride as I have some on hand. The so called bloom seem to come back as fast as I removed it from the substrate.
Downbeach, I am using carib seas oolitic sand. I was wondering to myself if by chance there was a contaminant, (silicate) that could be the cause from the substrate. I have a quarantine tank that does not get as much light as the display tank that grows these lovely pets, not as fast as it is getting ambiant light only from the display tank and the back room light when feeding time arrives. Hmm.:uhoh3:

 

[bertoni]

The bloom, whatever it is, requires phosphorus to grow. I would concentrate on that, personally. The sand might be leaching phosphate. That seems to be at least somewhat common.

The resin might be exhausted, or there might be an issue with the test kit, or the silicate might be high enough that the resin is doing the best it can. I might consider replacing the media and checking the silicate level with fresh media.

I might get a second opinion on the phosphate test kit. A measured zero is believable, though. If so, I doubt that lanthanum chloride will help. GFO might, over time, if the feeding level is low enough for the tank to handle.

 

[Arthur1]

Bertoni, thank you for the reply. I will replace the di cartridges by the end of next week and test the product water coming out of them for silicates. A side note, how dependable are hobby grade test kits? The "authorative" figure I spoke to seemed to want to drill me about how they're basically useless. Though the titration method of PO4 is testing zero along with a .03 on average with the photometer, a skim of algae does appear on the front of the tank within a couple days, signaling that there is available PO4 in the water. I will keep you posted! And thank you.

 

[bertoni]

You're welcome!

We see a lot of problems reported with hobbyist test kits, and I try to remember to suggest second opinions in cases like this. I might test some distilled water for silicate, for example.

As far as the phosphate tests go, organisms require fairly small amounts of phosphorus compared to carbon or nitrogen, so even a measured zero is consistent with higher phosphate levels than a reef.