Someone asked me the Benefit of a Reverse Undergravel Filter

Paul B

Well-Known Member
Someone asked me how a reverse Undergravel filter works and what is the advantage of it.
In the beginning, when we all kept bait, I mean fresh water fish we all ran undergravel filters. That was the only way to go and in fresh water they work perfectly.

When salt water started for home tanks in 1971 in the US (a little before that in Europe) some of us switched our tanks to salt but we kept our UG filters. Most of us did change the gravel to something different than the purple broken glass we had in fresh water and we removed the sunken chests and deep sea divers fighting sharks but there was no salt water gravel available.

After unsuccessfully using blue driveway gravel I discovered dolomite. You can still get it at a mausoleum or museum, maybe an archaeological dig in Egypt, I don't know but I assume crushed coral would also work.

I first ran my salt tank using a normal UG filter and in less than a year it crashed and I had to rescue my fish. There was no coral, live or dead rock then but we did have bricks, cinder blocks and roller skates. Much of my "rock" was asphalt that was dumped in the sea before I was born. I still have some of it and if you look close you can probably see remnants of the yellow line that was painted on it when it was a street. :oops:

I am not sure what the problem was by using a UG filter the normal way. We didn't have powerheads so they were all run with bubbles and they didn't run to fast but the salt creep on the lights caused us to have to turn on the lights with a stick because GFCIs were also not invented. As a matter of fact, to do anything on the tank we had to unplug everything and the only thing we could keep with success was electric eels. :rolleyes:

Anyway, the UG filters, after a few months became totally clogged rendering them useless similar to some politicians.

I decided to reverse the thing and instead of the water going down through the gravel, now it came up through the gravel. Something happened. It was a good thing. The tank didn't crash. It kept going and fifty years later it is still running.

Not crashing is a good thing but not the only benefit. I learned from Robert Straughn "The Father of Salt Water Fish Keeping" that the bottom of the tank is the perfect filter and the largest thing in the tank. Mr. Straughn used UG filters constantly but at that time, in the 50s he didn't quite understand the function of bacteria like we do today and he used the filter as a particle filter.

That works but you have to clean it constantly and as a whole, humans are lazy.
I discovered that if you pump water through the gravel at a slow speed and maybe strain it of particles first, the thing would not only last forever, 50 years anyway, but the tank would thrive and it would be easier to keep smaller fish.

A sand bottom has very little oxygen going through it as it is stagnant. But gravel, even if it is just sitting there has water flowing all through it. But if we give it a little help and push a little water through it, multitudes of creatures colonize it causing it to be a huge eco system.

Tiny tube worms, brittle stars, pods and bacteria completely fill every void. Those tube worms filter the water and the brittle stars remove particles. Very little detritus is left and a little detritus is good because it even provides more living space for those creatures which hate clean, sterile places to live sort of like Ozzie Osborn.

Those tiny creatures can breed in multitudes feeding smaller fish like pipefish, mandarins, dragonettes and anything that eats pods. I have many of those fish, they are all spawning and I never have to feed them.

This silly thing is the manifold I used for many years. I built a new one now but it is the same principal.
It is of course an old HOB filter. The three tubes coming out the bottom go to each of the 3 UG filter tubes.

The one on the left doesn't do anything and was a mistake, it is blocked.

Water is pumped into the thing from that hose on the left. I don't have a sump or I would have to divert some water from that to here.

I run about 250 GPH down each tube so about 500 GPH is pumped into the manifold where the water is evenly separated. Faster flow is no good, it has to be slow.

Once or twice a year I stir up the gravel where I can reach with a canister filter or diatom to remove excess detritus. If it was left forever it would probably clog eventually and besides, I like doing it.

Of course if your present system lasted longer than fifty years, do that. :cool:] [/URL]

Blue Space

Well-Known Member
way to keep it simple Paul. Like you said, 50 years and no crash... Not many Reefers even stay in the hobby for that long so that speaks for itself.

I think the old Engineering cliche maybe applicable... If it ain't broke, you haven't added enough features. :D
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