From: frazier@kona.cs.ucla.edu (Gregory Frazier)
Subject: Re: Wet Dry + UG reverse flow debate
Organization: UCLA, Computer Science Department
X-Newsreader: Tin 1.1 PL5
Date: Fri, 21 Aug 92 22:41:56 GMT

John Murphy (murf@spike.comet.ucar.edu) wrote:
: For the last 2 weeks I have been going from store to store seeking advice
: for a reef tank that I would like to set up.  I am planning on buying a
: 75G setup, w/ wet dry filtration.   One of the stores I have been to
: "highly reccommends" taking the return from the wet/dry and running   
: it reverseflow up through an undergravel filter.  Another store that
: I mentioned this idea to said that it was the worst thing you could
: do.  He said that anerobic bacteria would build up and gradually deplete
: the oxygen supply in the water.  He also gave several other chemical   
: reasons for this being a bad idea (he talked like a biologist) that I 
: didn't understand.  The advocate of the setup said how is bacteria going to  
: form when the water is moving up not down and it has already been through the
: wet dry filtration.   Both these guys have 15+ years experience with
: marine setup, but I have know idea which is correct.

WARNING - I am a freshwater aquarist, not a mariner.

They were both wrong, and on so many counts it is scary.
A biological filter converts ammonia to nitrate, via nitrite.
NH3 -> NO2 -> NO3, oxygen is being consumed.  The bacteria that
does this is termed aerobic.  There are also bacteria that
can take nitrates and convert them to nitrogen gas,
NO3 -> intermediate forms, including NO2 -> N2.  These
bacteria are anaerobic, and can only do what they do in the
absence of oxygen.

The most imporant job of your filter is to convert ammonia
to nitrates.  Thus, you pass oxygenated water over a lot of
surface area.  A UG filter gives you a *huge* surface area.
A wet/dry might give you less surface area, but it gives you
highly oxygenated water as well as the ability to pass the
water across the surface area at a high rate.  Wet/dries are
generally considered to be "better" than UG's, in terms of
their abilty to convert ammonia.  Also, in a marine tank, having
highly oxygenated water is important.

If you have a tank with both a wet/dry and a RUGF, I would
guess that most of the filtering would be done by the trickle.
By the time the water goes through the substrate, it will be
highly oxygenated and have little, if any, ammonia or nitrite
in it.  The oxygen would not get depleted, since there is no
ammonia/nitrite for bacteria to metabolize.  Similarly, because
of the high oxygenation, there will be no anaerobic metabolizing
going on in your substrate.

:   Does anybody
: out there have any experience with this????? which is best?  wet/dry
: nothing on the bootom?  wet dry + u/g and coral? wet/dry only and sand?
: what do I do?????????????????????????

The consensus on the net and elsewhere (I have no personal
experience) is that having no substrate is better for your
tank.  However, if you really want to have a substrate, putting
a reverse UG under it is a fairly good idea.  It will halp
prevent ditrius from accumulating in it.

--


Greg Frazier	frazier@CS.UCLA.EDU	!{ucbvax,rutgers}!ucla-cs!frazier