Interesting idea. Can't really see from the pics what size the openings in the shelf plates are. If too large, so that the gravel might slip through, you could put some plastic mesh on top of the plates. I used this stuff to keep my hydroponic media from getting through the eggcrate I used in my sump. It is in the sewing craft area, with needlepoint stuff and rug weaving stuff. It lays nice and flat. Mesh comes in two sizes, one about 1/8th inch and the other about 3/16th, I think this pic is the smaller opening. I used the larger one.
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Oh yeah, the holes are too big as is, I think I will use a heavy duty window screen, it is polyester coated in vinyl and pretty inexpensive too. I looked online at that embroidery fabric but it would take a bunch of pieces to cover one plate at 36" x 24"
I went ahead and ordered two of the 5 tier shelf units from Home Depot today and it was delivered today! Plus I get 10% off for being a military veteran, so with tax it was a buck seventy for TEN JUMBO Undergravel Filter Plates, only $16 per plate minus tax.
I was going to wait and hope for a sale price but then with all that tariff stuff going on, it is very possible that the price of goods made in China will increase very soon.
I got pics. These things will need very little modifying to be ready for service! Each five pack of Bottom Plates, er, um, shelf units, came with 8 lift tubes, 1.5" lift tubes! These things are gonna be perfect, and with the Czech Style airlift collar, these things will move a whole bunch of water, very exciting!

Aren't they Beautiful? And BIG!

Each five pack also includes 8 complimentary 1.5" airlift tubes! Notice the lift tube mounting socket in each corner, very convenient!

The modifications will be so simple, I hit a DIY Home Run with this idea, very exciting!
I fixed my bike too. It has two chains, the long primary chain from the crank (pedals) to the motor and a much shorter second chain from the motor to the axle.
Both chains are standard bicycle chains and I already broke two of the secondary chains and so I found out that some small motorcycle chains have the same length per link and so I bought one and sure enough it worked! I really don't think this one will break by the force of that electric motor.
Good Evening, Monsters!