----- Original Message -----

Hi all, just a quick question, is there any reason I should not use a fluidised bed filter in a reef and fish tank...

From: HL

Fluidised beds are great for FO tanks, but not very useful for reefs.  They work in a similar way to a trickle filter, but are much more effective.  Basically, a 12" long fluidised bed such as a Sea Storm 100 (which I used to own for a Cichlid aquarium, very good filter) provides 64 square feet of area for bacteria to live on.  However, like a trickle filter, they will build up nitrates because of the lack of anaerobic bacteria, such as you have in a deep sand bed.  In the end you would probably be much better off simply with Live Rock, Live Sand and a large skimmer.

I have recently returned to keeping Marine fish after a break of a few years, and technology seems to have changed considerably since I last kept a marine tank. My system is a 4 foot display tank, with a 3ft tank (outside the house) connected to it as a filter tank. (I can't really call it a sump, as it is higher than the main tank.) Total water capacity is about 60 (UK) gallons. It is Fish Only at the moment, and has been running for nearly a year. Water is transferred from the display tank to the filter tank through a 'Berlin Classic XL' protein skimmer, powered by an Eheim 1060 pump. The filtered water returns to the display tank by gravity. The filter tank contains only an undergravel filter with lots of coral gravel (old technology I know, but I didn't know any better at the time). I also have a "Rainbow Lifeguard" Fluidised Bed filter running on this filter tank.

Now for my question... I have seen a few messages on this forum which suggest that Fluidised Bed filters are the cause of high Nitrate levels.  I do know that the large volumes of bacteria that are supported within a fluidised bed are very efficient at breaking down Ammonia and Nitrite into Nitrates, but surely they will not produce any more Nitrate for a given amount of Ammonia/Nitrite than any other form of biological filtration - OR have I got this wrong? The reason I ask this, is that I have a very high Nitrate level, and it would be very nice if I could slow down the production of Nitrate simply by removing the Fluidised Bed and just relying on the undergravel filter.  Then I could concentrate on actually lowering the Nitrate level by various means, before introducing inverts, as I would like to do once I solve the Nitrate problem.

So would there be any advantage in removing my Fluidised Bed, or would the undergravel filter produce the same amount of Nitrate (assuming the Bio-load remained the same), as I suspect is the case.

As you might recall, I am the guy complaining about fluidised beds. I would like to point out a few things, that I have done, which has helped, but first I will explain why I PERSONALLY think fluidised beds are more trouble than they are worth. Brace yourself, this is going to be long!

OK, to give you an idea, my tank is a 100 gal FO, (soon to be modified). It has about 25" of fish stocking density at the moment; all fish are between 2" and 4". Filtration is a Berlin Classic, and the same fluidised bed you have. I do 25% water changes weekly and still can't get nitrates down. This annoys me since I know quite a few people who have much higher stocking density, do less water changes, and have much better water quality ... only difference being the two systems filtration ... wet/dry vs. Fluidised beds (FB). This is not just isolated occasions... I know folks who have the same set up as me, with the same problems as me, and folks with wet/dry's who have nothing near the problems. Logic deduction ... cause = FB.

You will read on just about every website around how FB's are notorious for this nitrate problem. The reason usually is believed to be as follows: You are correct that surely the same amount of nitrate is produced, because after all it is the result of the exact same cycle, the difference is how it is produced, and what happens after that. First, the FB produces nitrate really quickly, and any nitrate reducing bacteria in the tank cannot cope with this high rate. Why this makes a difference I do not know, but there is loads of scientific jargon out there to support it, so I take it for granted. The second thing is that a fluidised bed has absolutely ZERO capability to cope with nitrate... by design it is 100% aerobic, making it impossible for nitrate reducing bacteria, but excellent for the other guys. On the other hand, once established, a wet/dry often develops areas of low flow etc, where a small amount of anaerobic bacteria can grow, and get to work on the nitrates. This is why I believe FB's are so bad... because they are so efficient at what they are meant to do.

I know this is a very rough, cagey description, but basically that is what a hell of a lot of people think, and since I do not know better, I go along with it, because it makes sense. In my tank over the past year or so I have come to the following conclusion about nitrates and fluidised beds: It seems logical that if you can remove rubbish before it goes through the nitrogen cycle, then fewer nitrates is produced. Now, I used to have my system so that the fluidised bed ran off the main tank, and the skimmer ran off the sump. So basically, the skimmer was receiving "raw" water, and so was the FB. So the water that the skimmer got to first had a percentage of dirt pulled out, that never got to the FB. Hence a reduction in the total possible nitrate produced. Then one day I decided I would put the FB in the sump, boost flow through the sump, and let it all run together. Now my sump is quite small... and basically it was arranged so that water from the tank entered, and the skimmer and FB were just placed in the sump. "Raw" water, and water that just came out of the FB and skimmer got all mixed up and so on, then was returned to the tank. Basically, a lot of the dirt that the skimmer used to catch first now got to the FB first ... hence more waste entered the Nitrogen cycle, hence more nitrates produced. I also noticed a marked reduction in skimmer waste produced.

So, back to the drawing board! I now have a sump that functions like this. Dirty water comes in. A percentage of this water is taken away to the skimmer. The return water from the skimmer then enters a chamber that feeds the FB (this is such that ONLY skimmed water can ever reach the FB, and "raw" water direct from the tank never gets into the FB). The FB then returns this to the main sump where it is pumped back to the tank. This is also arranged so that water just coming from the skimmer and FB cannot mix with the feed water to the skimmer. The result has been that once again my skimmer is producing A LOT more scum. I still do weekly 25% water changes, but now instead of my nitrate rising by about 5ppm per week, it is falling very slightly per week. Previous to this new sump design this amount of water changes wasn't enough to even keep nitrates stable ... now it is enough to slightly reduce it. That is a very good improvement by just re-arranging my equipment.

Yet still, despite this high tech arrangement, my system does not operate as efficiently as those run on the wet/dry filters. I have always believed the FB to be at the root of the problem, and now more than ever I am convinced. Here is the reason why. I decided to fill my canister filter with biological medium (Eheim ehfisustrat ... 5 litres of it), and use that to help with filtering. So I filled it up, let it settle for a few days, then turned down the flow slightly on the FB, so that the sand rises about 1/2" less than the optimum level. Over a week or so, as the canister begins to mature, I turned the FB down further. This brings me up to date, to today. The FB is now running with a flow about half way between minimum and optimum (as indicated by the stickers on the unit). Here is the good bit. The Nitrate level is now down by half! All along I struggled to keep it around 50ppm. I still do the 25% water changes, but now instead of barely managing to maintain a level, it is reduced by about 10ppm a week. It is now hovering just above 20ppm! The only significant change is that the canister is doing half the work, and the FB flow has been reduced by half, just like the nitrate level! To my mind, the only reasonable explanation is that removing the FB has had a hugely positive effect, and thus it must be the root of the trouble all along.

I am just about to receive 20 kg's of LR. Once I cure this and get it in the system I will remove the FB altogether. If my assumptions are correct, this will mean I have a very low nitrate level quite soon, due to the water changes that I intend to keep up for a while. I eventually aim to be able to maintain less than 5ppm nitrate with about 10% weekly changes. Many think it is impossible, but I feel it is achievable, and have heard accounts of systems very similar to the one I hope to have, which have zero nitrate. So, my advice to you is to try and arrange your skimmer and FB so that only skimmer water gets to it.

I think you may be right about the fact that nitrate can only be produced if there is nitrite (gut feel though, no proven facts). Still, you will always have ammonia and nitrite as long as there is living animals or decaying matter in the tank. So, IMHO, do not try to stop the nitrate build-up by removing all your aerobic filtration. Rather try and add a "nitrate reducing factory" to work in conjunction with your "producing factory".

Having said that, you should try and get the system to balance. Rather stop the UGF, and continue with the FB for a while, or , as MS suggested, change to a Wet/Dry filter. You can just stop water circulation to the UGF, vacuum the sand once properly, and then just leave it be. The UGF plates make quite a nice refugium for your sand living organisms in time, and could conceivably act as a mini Plenum as well, so no harm in leaving it in the sand.   Now, get some live rock, and if possible, a live sand "starter culture". Either would cause your sand to become "live", given time.

With the sand bed at least 3" thick (4" would be better), and some live rock, your nitrate level should start to fall. After a month or two, you can try stopping the FB as well. If the ammonia or nitrite starts to increase, you will know that you still need an aerobic filter, and so you can just re-start the FB. (It would be a good idea to keep it running in a hospital tank, or any container with old tank water, during this period, so you won't kill the aerobic bacteria while you're experimenting). Just keep in mind that whatever you do, you should do it SLOWLY. Don't stop the UGF and FB the same time, start the Wet/Dry at least a month (two would be better) before you stop the FB. Let the system stabilise for a month or two after each change, before starting something else....