By Bob Goemans
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Bob Goemans corresponds with Robert Barcroft

Robert Barcroft writes...

Dear Bob,

I am thankful and appreciative of all the information you have on your website and for all the good advice you have given me over the years through your column in FAMA. I have now had a plenum system for over seven years and for the most part have had good results. But am now starting to have a hair algae problem that does not want to go away. I changed to R/O & D/I water with little effect, and added phosphate removers with the same result. Someone then suggested there is an accumulation of nutrients in the bed and I should break down the aquarium and clean all the substrate, clean out under the plenum, and then set it back up and that would fix the problem. Is it necessary to do this from time to time? Any help you can give in this direction would be appreciated.


Robert Barcroft

Wadsworth, Ohio

Bob replies...

Hi Robert,

Thanks for your email, and there are several aspects needing discussion. First, and foremost, the plenum bed itself needs monthly maintenance (as does any sandbed). All sandbeds are detritus collection areas, and if not vacuumed will continue to accumulate detritus and become a source of nutrients for unwanted algae or other problems. (Would people never dust their home?)

Plenum beds also need occasional surveying for clogging, as fast forming bacteria colonies or precipitation from improper alkalinity and/or calcium additives can cause sand particles to adhere to each other and form hard chunks. Use a butter knife and run it in a criss-cross pattern throughout the open bed surface areas. If chunks are found, break them up with your fingers.

If these aspects have not been cared for properly, then its possible the bed needs to be removed, cleaned, and reestablished. But I know of plenum beds that are still operating magnificently after 12 years of use! And I had one plenum system where the aquarium's contents were simply moved to a larger system and the plenum (area under the grid) was spotless when it was removed after six years of use.

Then there's overfeeding and of course, overcrowding, which if not corrected, can elevate the system's phosphate and nitrate levels, making it an ideal environment for unwanted algae. As for phosphate, it should be maintained at or under 0.015 mg/l. A quality brand phosphate media, which should be used in a canister filter or reactor, should always be in use.

Possibly some small sea hares might be a way to quickly cure your algae problem, but their waste also needs to be removed, as does the majority of waste of all the critters in the tank. Unless the reason for its growth is eliminated, the algae will come back.

Hope this helps,


Dear Bob,

I finally found a phosphate test kit in a local fish store (30 miles away) and promptly purchased it. I tested the aquarium and found phosphate was below .2 mg/l. In fact it did not show any blue at all at least that I could see. But when I tested the source water in my reservoir it was more than 1.0 mg/l. So it seems the algae growth is the only thing allowing my corals to survive from a phosphate overload.

I know this problem needs to be eliminated, but currently do not have the cash on hand to replace my RO membrane that is clogged and my DI canister. By the way do most membranes fit in other housings, as do cartridges in other canisters? Currently I am using a Kent phosphate sponge in my Whisper 20-40 hang on the back filter and that is all that is in it. I would like to know if PhosBan is the better product and also if PhosPure by Drs Foster and Smith is just as good. Also, I think it would be good to try and remove the phosphate before it enters my tank so am thinking I could use PVC to construct a tube filter to house a bag of PhosBan with a powerhead to force the water through it. Any thoughts you might have are always appreciated and always invaluable.

Thank you so much,

Robert Barcroft

Wadsworth Oh.

Bob replies...

Hi Robert

There are good phosphate test kits and then there are some that are 'excellent!' The Merck kit is what I'm using, as it's extremely accurate, yet more expensive than the average aquarium phosphate test kit. But when dealing with this compound, i.e., phosphate, the cost difference may be worthwhile when related to the value of the animals in the aquarium! And with the Merck kit, you will get a defined reading! Worth the extra cost in my opinion.

And no, most RO/DI filter replacements are not interchangeable, especially on the better brands. And there are average RO's and DI's, and there are really very good RO and DI equipment. Suggest comparing yours to the SpectraPure models, which are reviewed on my website.

As for phosphate removal, there's a right way, and ways that are inefficient and wasteful when it comes to time, effort, and their cost. Here's how I do it - I use a Fluval 302 canister filter - I first fill the lower basket with activated carbon (I use either ESV or Boyd Chemi-Pure) in a sack of course, and I also cut up a Poly-Filter into small pieces (More areas of absorption that way). That fills the first basket. In the middle basket I place a layer of filter fluff and spread five tablespoons of ROWAphos or Warner Marine phosphate remover medium over the filter fluff. Then cover that media with another shallow layer of filter fluff. I do the same in the top basket and change out all the media when there is the first sign of any phosphate reading on my test kit. Nevertheless, I change out all the media every couple of months since my aquarium is well established.

Phosphate sponge filters are better than nothing. PhosBan is a good product and again, better used in a canister filter or Phos reactor. I can't speak for PhosPure, as I've never tried it, but from what I understand, it's the same ingredient that is used in other quality brands.

The PVC filter is a good idea, however, a single pass through it may not be enough to significantly lower such a high phosphate level. You might have to pass the water through it 'several' times. Once the makeup water is cured of phosphate, then begin to lower the aquarium's level, as your goal there needs to be below or no higher than 0.015mg/l.

Hope this helps,



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