Quote:
Originally Posted by Myka
What you're describing is common phenomena. The cyano bloom happens in our tanks from GFO for the same reasons it happens with biopellets - quick, significant lowering of phosphate. In other words, too much GFO (or too aggressive with biopellets). It can happen with Lanthanum chloride (FozDown) too for the same reason. It doesn't happen in every tank though under what appears to us to be similar circumstances which is obviously frustrating. Just as frustrating as high nutrient SPS tanks that get no waterchanges or dosing and grow SPS like weeds for years and years (lol). Anyway, the reason the cyano appears is because the water column is suddenly barren of phosphate yet the substrate and rock is still loaded. As the phosphate leeches out of the substrate and rock the cyano jumps in to feed off it as it leeches. Cyanobacteria are extremely efficient at processing phosphate.
The fix: Pull the GFO. Keep using carbon. Add a filter sock, then use a piece of rigid airline tubing with flexible airline tubing attached to make a mini siphon hose and suck the cyano out of the tank at the peak growth of the day (usually at peak lighting). Do this daily or every second day. After you suck out as much as possible use a turkey baster to blast every nook and cranny in the rock. Then use a feeding stick or the baster to stir up the top 1/2" of sand. Wait an hour and remove/change out the filter sock. It takes elbow grease, but it's very effective.
Next time, don't use so much GFO. 1 tbsp per 30 gallons changed out every 2-3 weeks is much "safer" than adding a whack and changing it out every 6-8 weeks.
Added note... in my experience, mature tanks shouldn't need GFO, and usually do better without it.
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This is the common wisdom I'm suspicious of. There's never been any evidence or experimentation to show that this is true, it's just what people have surmised because they couldn't explain what they were seeing. But when you start to dissect it from a biological perspective, the notion that suddenly lowering phosphate levels would cause cyano bacteria doesn't make much sense.
1. If cyano is so good at taking up phosphate, it should do as fine a job pulling it out of the water column as it would out of the rocks. It wouldn't not grow when there was plenty available for cheap and easy in the water, then suddenly start growing when the easier to get at stuff disappeared. Chemicals diffuse along gradients, so cyano taking phosphate out solution directly above the rock it sits on top of will create micro gradients that phosphate can move across, regardless of the concentration in the water column. Further, there's good evidence to suggest cyano biofilms have multiple mechanisms that would allow them to actively harvest phosphate in the rocks they sit on top of (micro pH gradients, oxygen gradients, CO2 gradients, direct microbial weathering, etc), which is probably why it seems to favor recently exposed coral skeleton, as that stuff is loaded with phosphate. I can't think of a reason why reducing phosphate levels in the water column would enhance that activity that holds up to close scrutiny.
2. We like to think gfo reduces phosphate levels to "zero", but the truth is that even the most heavily gfo filtered tank water still has as much or more (usually MUCH more) dissolved phosphate in it as most wild reefs. On average it keeps it low, but phosphate levels moment to moment fluctuate in a tank (ie, right after feeding), even if the long term average is "low" due to gfo. If corals and coralline are growing, and you have to wipe your glass down ever, there's enough phosphate to support cyano, regardless of what your test kit tells you. I can also go on and give you reasons why hobby grade (and even some lab grade) phosphate test kits are pretty much never accurate within the range relevant to reef life, but that's another rant.
3. Cyano bacterial biofilms, which can achieve staggering amounts of surface area where 100% of the tanks volume flows over them every few minutes in the immediate vicinity of where phosphate (aka food) is added and broken down will almost certainly be better competitors for nutrients than a teeny tiny reactor in the sump, fed by a teeny tiny pump with flow throttled so the media barely bounces. Over time it brings down the phosphate level for sure and maintains it there on average, slowing the growth of algae, but that doesn't mean lots of other organisms don't have a chance to get some of it first.
Basically I'm saying I think that explanation has a lot of holes in it, and it's never been based on any evidence, or even a biologically reasonable theory. When you add a substance to an ecological system, and suddenly massive growth of a previously absent or minor organism occurs, the simplest (and therefore usually true) explanation is that that substance was a previously limiting nutrient for that organism.
We twist ourselves in knots trying to explain things we see in aquariums by invoking complicated "balance of the system" explanations that survive because they fit well within the narrative we tell ourselves about our tanks, and are virtually untestable.
The more simple answer is that even in a bio pellet or gfo treated tank, there was and always will be enough major nutrients like N and P for certain problem organisms like cyano grow, but that cyano can directly use the organic carbon or rust particles (or both) sloughed off in to the water column as food or an otherwise limiting nutrient.
In the case of gfo, there's plenty of scientific evidence to support the link between iron feeding cyano, but virtually no evidence to support the line of reasoning most aquarists have used to explain this sort of thing.