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Undergravel filter - gravel size


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Hey all! I am looking to set up two under gravel filters and I was looking to use lava rock pebbles as the media/gravel for it. It is to large to fall into the plenum, but is there such thing as a gravel size that is to large? I know too small can clog on an UGF but will gravel that is to large defeat the purpose? 
 

thanks! 

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I kind of wonder if you are going to have problems with food falling b/w the large rocks and rotting away but that would happen even if you weren't running UGF.    My son's glofish tank has Caribsea Snowy River gravel and that gravel is quite sizeable with smaller chunks mixed in.   The hard part I had was positioning the smaller gravel chunks to hide the filter plates and to fit b/w the glass and uplift tube.   Since your link is showing pebbles all the same size I do think you may have problems fully hiding the plates especially where space is tight b/w the glass and UGF parts.    Glofish tank has only been up a week (hasn't even needed a water change yet) so I can't give you any feedback into longterm success with UGF and chunky gravel.  Truthfully with how big the gravel is I'm worried about the rotting food situation in his glofish tank but my son insisted on this gravel so we will see.   

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What you're looking for in an undergravel filter substrate is surface area. More surface area gives more places for bacteria to colonize giving you a larger pool of bacteria. Lava rock being somewhat porous does give you decent surface area. Sand gives you lots of surface area. (Which is why it's used in fluidized sand bed filters.) Bigger, nonporous pieces of substrate give you less surface area. I've seen reverse flow undergravel systems used in marine tanks where the water flow is so high that the sand bed of the tank is at least somewhat fluidized. This can create issues with sand being overly mobile and sucked into filters and damaging pumps. 

If you're going to understock the tank, it doesn't really matter what you use. One highly spoiled betta in a 125 gallon tank won't matter. If like most fishkeepers you're more inclined to overstock the tank, then you'll want a finer, more porous substrate that gives you more surface area for bacterial growth.

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I use the med size pea gravel from the nursery I just rinse it well on a large black garbage bag cut now opened up in my driveway slopes so the smaller stuff close down the driveway and the bigger stuff I let dry in the sun that way it’s pretty clean the sun pretty much kills anything on it and then use it in the underground.Sand fine materials just go right through it. My experimental son has used a peat with sand topper. It works great now. “It’s 2months old”. Kids but I want to to continue and his young 

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I have a lava rock substrate in my 120 gallon and it works fine. You linked to home depo pebbles, but they are probably going to be too big to be practical. I used 1/4" lava rock gravel that I bought from Bonsai Jack. I used a sieve to get rid of the dust and particles smaller than 1/8".

I am considering adding and under gravel filter to my tank. That will be a project, but I have already ordered the under gravel filter.

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This is also how many folks would run sand with a undergravel filter back in the day.

 

On 8/25/2021 at 11:29 AM, Ben_RF said:

Back in the day, to help prevent small pebbles and mulm from building up in the bottom, we would take fine mesh or cloth and put over the top of the undergravel filter.  I am not sure how effective or if it actually impaired, but thats the old hack many folks use to use to help keep things from building up down there.

 

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