Pumice or any other smaller graind lava rock is also often used.. Me to i want to make a hydroponic filter system for my 200 liter small terrace pond this summer. Still got a bag with akadama x pumice mix 50/50 which i want to use as cap layer, with some lava rock as base substrate. Already looking up information for this idea for 2 years now, but yet didn't make any setups.. Doesn't realy mather what substrate you use as long it has a good drainage.
Actualy it's all relatively simple there you have 2 choices a constant flowing system or a wet and dry system. The constant flow system speaks for itself it's filled as fast as it drains again and goes constantly round.
The most effective is the wet and dry setup where the system gets flooded till a given level and drained again. This gives plants more time to take up nutrients and in filterring perspective the most effective because the flood and drain concept also gives the bacteria more time to convert wast products..
For the wet and dry system you would need to make a bell syphon which also is relatively simple to make concept and because it works with the forces of nature a very nice learning project for the kids and maybe yourself, to build and see it function.
Very simple and explained in 46 seconds
Ofcourse you would need a pump on a timer which for example does cycle it 4 times per hour.. i didn't ask but i assume you want a tank or little pond what ever with fish etc. to provide the ferts for the whole setup. Than you need to take in consideration the size of the filter and the size of the tank were the water comes from. When filter floods the tank drains obviously. So the hydrophonic can't be to big, you need to do some math with how much water is acceptable to take out the tank and go with that volume for the filter setup.
For the rest there's nothing much to it and very simple..
I'll stay tuned and like to see what you and the kids come up with.. I would say for you stay tuned as well if you llike.. I'm definitely going to build mine this summer and certainly will make a little journal about it. And i realy want to try the bell syphon, because it so much more fun.
Good luck..