Not sure if you remember me but I was exchanging e-mails with you earlier about moving with an algae attack... well we've moved and the new tank is set up in the new house and I moved the plenum outside of the main display tank to a 2' by 3' dedicated plenum. I have a little more work on getting a frame placed around the tank, and when that happens, I'll be taking some pictures and I'll send them to you.
So far everything seems to be going as I'd expect, diatoms started on the glass and I'm monitoring ammonia (basically treating it as a new tank, which it is)...
Anyway my question is about the plenum, I have this huge plenum with nothing in it. (Its connected off the sump, water comes from the sump and goes back to the sump then to the main tank). I know you mentioned that it was an ideal place to raise sponges, but where do you get them? How old should the tank be before trying to get them? Should I put snails or starfish or anything else in the plenum? If the answer is no then that's fine but if I can take advantage of that space for growing white shrimp for the tank or snails, or if some starfish would help the plenum then I just wanted to know.
As always thanks so much for your help.
Douglas P. Milone
Yes, I remember and bet you're glad the move is over! As for your questions, the plenum is there to provide 'excellent biological filtration.' As for the area serving as another aquarium to raise various kinds of livestock, it's not a refugium. Its sole purpose is to change the majority of its sandbed to that of having an anoxic process, thereby efficiently degassing nitrate, not producing ammonium, as what happens in the depths of deep beds on the bottom of aquaria. Nevertheless, I found sponges, which are also great filtration devices, to do very well in this type environment, as they only need good current and shady areas to grow well. Yet getting them is somewhat difficult at times, nevertheless, some of your local shops may be able to order some for you. Another possibility is online shopping at some of the major aquarium suppliers. And when you do get them, raise them up off the bed surface with some shallow height eggcrate platforms. Keep in mind, do not allow them to be exposed to air when being removed from the shipping bag, as that may kill it. In fact, when purchasing new specimens, they should be bagged under water, i.e., never lifted out of the water. Nor should any air or oxygen be added to the bag, as it may become trapped in the animal's tissue while being transported and lead to its early demise. Furthermore, new specimens should not contain any gray or white tissue, which is generally a sign of dying or dead tissue. However, it is possible to cut that section out, of course under water.
Keep in mind many types of hermit crabs feed on not only some detritus/alga, but also the bacteria that are attached to the upper sand grains. So I don't recommend too many, especially in new aquaria and none in the plenum unless some sort of diatom problem is developing. As for snails, if not needed to control slime or short stemmed algae, they too are not needed in large amounts. A few, yes, many no. So six snails, not turbos, would be ok in your plenum. As for raising anything else in the plenum, consider it the heart of your filtration system, and keep it as clean as you can.
I'm attaching a photo of my plenum bed, and every little outlined area is nitrogen degassing and working its way upward in the sand column. I even missed some areas, but you can see just how efficient plenums beds are!