Substrate as only bio filter?

  • #1
I have a 20 gal long tank with an under-gravel filter hooked to a aquaclear 70 powerhead. I have the aquaclear pre-filter attached to decrease water cloudyness. The output of the power head is being piped into a clear tube where I have 2 large plastic mesh walls to create turbulence / even out the flow through the rest of the tube. The 20" tube has acted as a fish treadmill, and I have really enjoyed it. However a few hot days, and lack of knowledge have killed off all but one of my fish, and I am considering fishless cycling if the last one dies. So here is the question:

With good water flow can the substrate be used as the main / only biological filter? From timing the flow I am getting at least 2.5-3 gal per min from the power head, and looking under the filter (yes the tank is heavy) it is staying pretty clean. Before the AC going out and the tank temperature going up 10 degrees f, I had dropped to 0 ammonia 0 nitrite 30-40 nitrate, the mass death of the heat shocked and partially nitrite poisoned (from the tank cycling) fish unbalanced everything.

The only fish I am keeping in this tank are some white cloud mountain minnows, so I am also considering building them a small cooler, and keeping the tank at a much cooler range. Any guesses on what 65 degree water would do to the bio filter, holding more oxygen but being colder?

Thank you all in advance!
  • #2
Welcome to FishLore!

Substrate will house enough bacteria to keep a tank cycled but...

Undergravel filters can be pretty messy. In time a bunch of pretty disgusting stuff will build up under the plates. So you will be forced to drain the tank, remove the gravel, lift the plates and clean the disgusting gunk out from under them (I recommend doing this before eating!).

If it were me, I would remove the undergravel filter now and get an appropriately sized AquaClear filter or a cannister filter if you feel like spending a little money. I like the AquaClear filters because the sponge, carbon and ceramic biomedia are all separate and it makes it real easy to clean the sponge and replace the carbon (much like a cannister filter).
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  • #3
I have about a year before I should get too deep into extra equipment, at which time I will probably set up a larger tank, with sump, and all sorts of other ... my wife would call it craziness. I have all ready realized from my reading that an UGF can and probably will be more trouble than it is worth. In the future I will likely use one as a pressurized way to help keep my substrate clean (reverse) but right now the UGF intrigues me. I will be building a stand for the tank in the next month, so I will make sure I can see the bottom from underneath. At least that way I can monitor the build up.

Once the tank is fully cycled, how delicate will my substrate / bio filter be? Will I be able to drain part of the tank water into a bucket, put my gravel in it, drain the rest of my tank, clean out under the UGF, and put it all back, and still be cycled? Would my goal be to over populate the bacteria by stirring around my gravel, and getting to the point that it can fully break down 5ppm ammonia every 6-12 hours?

I expect my last fish will die any day now, and if it doesn't I may just transfer him to a smaller tank while I fishless cycle this one. I feel bad for him being all alone. I would bring him back to the aquarium store except he has gotten use to my 8.2ph water.
  • #4
I agree with jdhef. If your last fish dies you should ditch the undergravel filter. I think they cause more problems than they solve. Even though you are getting adequate flow through it, I don't feel it will be efficient enough to be your only means of filtration. You can get an aquaclear 30 for $25.00 off amazon.
  • #5
Part of the reason filter media is so effective as biological filtration is because it's porous and and therefore can contain a lot of bacteria. Substrate isn't porous and so can hold exponentially less. I would switch to an HOB or a canister filter.

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