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Fluval G3 g-nodes very dirty??


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Howdy, I have a Fluval G3 filtration unit on my Fluval M60 (25 gallon tank). it has a filter cartridge that is to be cleaned every 30 days. I have had the unit for 18 weeks and have cleaned the filter about every 25 days. I have one small fish and I dont feed much, so there is not a big bio load, nor do I have algae problems. The question I have is that the g-nodes look really dirty. i was surprised at how dirty they were (please see picture). I have struggled with high Nitrates (40ppm) even before I had the G3 filter, but I really expected the G3's Nitrate cartridge to reduce or eliminate the Nitrates in coordination with a Phosban 150 running Katalyst denitrator pellets, but this has not been the case. i recently reduced the depth of my sand bed as well, its about 1/4 inch at this time. Does anyone else have one of these G3 or G6 units and did your g-nodes look like this at any point?


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Here's my take and please keep in mind, my experience with both the medias you use is non-existent, as well as the filtration unit itself.

First, your katalyst denitrator media sounds like biopellets to me from the description from the manufacturer. To be truly effective, this needs to be combined with an adequate skimmer to remove the bacteria after they use the carbon source from the media and uptake nitrates and phosphates. Once the bacteria are exported from the system, then you've essentially removed that net amount of nitrogen from your system.

For the g-nodes, the manufacturer says it helps with ammonia and nitrates. While a nice statement, it's mainly going to really do one or the other depending on the amount of flow over the media. Main reason is the amount of oxygen supplied to the nodes. In a filter setup like yours, there will be ample oxygen supplied to the nodes so you're basically bolstering ammonia and nitrite processing via nitrification. If they were put in a much slower flow, it can potentially reach suboxic conditions and allow for processing of nitrates via denitrification. I'm talking almost no flow so the filter setup is not ideal at all, even on its slowest setting.

In summary, you're probably getting no help from either media with your nitrate issues.

To remedy that, I'd make sure I had a good skimmer and route the effluent from your katalyst media to the skimmer input. Or, I'd relocate the nodes to a stagnant area in your sump with little water flow.

As to why detritus is building up on your nodes, they are acting like a physical media catch, basically like a large filter sock, so that anything in the water column will be trapped in them. Then bacteria will probably grow and you get that stuff you see all over your nodes.

Believe it or not but your nodes may actually be adding to your nitrate level issue. When you increase your tanks ability to process ammonia and nitrite even faster, you end up adding to your nitrate level. Randy Farley Holmes had a really good explanation of this phenomenon which I can't recount at this time but I thought the statement shocking when I first read it myself.

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