Silica dosing related to bleaching event?

bpcass

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After reading all of the articles, posts, and anything else I could find on the topic I decided to try silicate dosing on my 75 gallon reef. I read Randy's article particularly closely, confirmed the normal concentration range for ocean silicate through multiple sources, and purchased sodium silicate from the Science Direct site that Randy linked to. I then performed all of my calculations and test dosed a 10 gallon macro algae tank to ensure that my calculations were correct. Within that tank I achieved a silicate concentration of just below 1 ppm confirmed by a serra test kit 6 hours after dosing. Silicate fell to undetectable levels over the course of 3 days. I repeated the dose and had the same rate of uptake, saw no ill effects on the macro algae, pods, or mushroom corals present. I saw a small increase in growth on my glass that I confirmed by microscopy to have an increased proportion of diatoms compared to pre-dosing glass scraping samples.

Feeling I had done my due diligence I decided to go ahead with dosing the tank. I confirmed 0 silicates prior to starting and sampled my glass finding few diatoms. I added my calculated dose and tested the next morning. My silicate concentration was only 0.5 ppm, less than expected but I attributed that to a combination of a conservative rounding error in calculating system water volume and possibly more rapid uptake in a more nutrient rich tank. After 4 days silicate levels had decreased to undetectable and all of the organisms appeared fine except for a tiny amount of bleaching on one of the tips of my montipora digitata. I confirmed increased diatom growth and was very pleased. I thought the bleached tip might be related to fish damage or having recently gotten a little behind on top off and had hurriedly added 2.5 gallons of cold top off water the day before. I dosed the tank again. When I returned from work today the montipora was completely bleached! The other montipora frags have some patchy bleaching but have not gone completely. I performed water testing and found the following.
Temp 79
salinity 1.026
pH 7.9
calcium 400
alk 8 dKh
silica 0.5 ppm
nitrate 0
phosphate 0

I am preparing to do a water change and have an extra purigen pouch I can put in the sump. I also have an emergency polyfilter pad that I could use. What are anyone's thoughts? Is this bleaching likely related to silica dosing? Should I have started slower? The only other unusual changes for my tank were the large top off, I missed feeding the tank the night before, and I am about a week late on my bimonthly 10% water change. The tank has overall been healthy with a mild hair algae problem and good coral growth for a mixed reef of mostly LPS, softies, and zooanthids. Montipora is my only SPS. The only other coral having problems is a fungia that got damaged by too much flow and has been struggling for a month or two to recover. I was dosing silica to hopefully boost pod growth and suppress the hair algae. Could the growth of diatoms have dropped my nutrient levels too quickly for the montipora to adjust to? Would there be any point in using the polyfilter pad? Any advice for salvaging the coral?
 

Randy Holmes-Farley

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I have a hard time believing the silicate itself was the issue, but it might have been contaminated with something else that was a concern, or the bleaching was unrelated to the dosing. I've not heard of anyone report such a link.

For a contaminant in an additive that itself is only added at 1 ppm, a toxic impurity would either have to be present at a very high concentration in the silicate solution, or be super toxic.
 
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bpcass

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I'll do the water change and purigen. Do you feel it is worth it to try the polyfilter?
 
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bpcass

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Thanks for the help Randy. I'll post updates to let you and others know how it goes. There's relatively little info on silica dosing so I'd really like to figure out if this was related or not.
 
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bpcass

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No further dosing, silicate level undetectable. Montipora appears as though it may pull through. Some color, no polyp extension, but at least no algae growing on it. I will watch it for a few weeks and see if it makes it then restart at 1/4 or 1/2 dosing and see what happens.
 
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bpcass

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Close to ready to attempt dosing again. Since last update have seen monipora health improve but not the level of polyp extension or growth I was seeing prior. Made a major upgrade today by adding a Tunze Osmolator ATO going into a DIY kalk reactor. Reactor is just a Coralife super skimmer. I am adding my RO through the skimmer outlet which is an external drop tube to the bottom of the skimmer. This puts the infeed water at the level of the kalk sediment, output comes from a tube in the middle of the water column. Kalk powder can be added from the top through the skimmer neck. If all goes well for a few weeks I will try dosing silica again.
 

Randy Holmes-Farley

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One thought I had is perhaps increase diatom growth reduced nutrients too far too fast for your corals, and caused the bleaching.

Are you monitoring nutrients?
 
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bpcass

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I considered the same thing. I have had undetectable levels of phosphate and nitrate based on API test kits for nitrate and both API and Salifert kits for phosphate, but I had hair algae, bubble algae, and a small amount of cyano growth. Since my first attempt I have replaced my Hydor circulation pumps with a pair of Jebao wave makers and added a BRS GFO reactor, very slowly increasing the amount of GFO used. This has caused a substantial improvement in my algae problems but I still have some. I believe the GFO will strip out some silicate but still think I would like to dose in hopes of boosting my diatoms and thus my microfauna.

I hope that the ATO with the kalk reactor will significantly improve my stability and raise my average pH levels. Previously I was dosing kalk in my top off water that I would add every 1-2 days. Between dosing my pH was generally 7.8 to 8.0. Since adding this system my pH has been 8-8.2 whenever I check.

I'm going on vacation in mid August and plan to try dosing again after I return if all continues to go well.
 

Randy Holmes-Farley

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Did you notice a shift in the color of the algae growing on the glass from green (green algae) to yellow/brown (diatoms)?

I did, and show pics here:

Feature Article: Silica In Reef Aquariums ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/2003/1/aafeature1

from it:

image_preview


Figure 3. A view of the inside of the front glass of my reef tank 5 days after scraping the glass.

image_preview


Figure 6. A view of the inside of the front glass of my reef tank 5 days after scraping the glass and adding the silica supplement.
 
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bpcass

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I did notice a change in the test macroalgae aquarium, but I don't think I dosed long enough to get a visible change in my main tank. I have a microscope with a camera and did see an increase in the proportion of diatoms from debris washed off of my mag float following dosing, but that sampling method is not very consistent for quantitative measurement. Too many variables. Do you have any suggestions on how to use my microscope to get a more conistent method for quantifying diatom growth? I thought about using a cotton swab to sample a 1 inch swipe from the same area for each test, but feel that there would still be inconsistency in preparing slides.
 
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bpcass

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I could hang a slide in the tank using a veggie clip for a set amount of time and count diatoms per high power field.
 
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