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@Torrey No need for light with both methods i suggested and the 2nd one you don't ever put it in light but get food that can feed the smallest of fry. They wont get you green water to start with but WILL get you infrasoria which are microscopic organisms that fry will feed on.. once you have that, if you put it in sunlight and you'll get green water.. I use infrasoria to feed Betta's I bred before and my Rice Fish fry now. To get green water you dont need strong light, just light. You can also use a little 5w LED light bulb. Just make sure its cool white, not warm white. If you get a jar of aquarium water, add a crunched up lettuce leaf then put near light, even just the light from next to a lighted aquarium.. after a few days when it goes cloudy, add small amounts of very finely ground up fish food.. just a little bit.. that too will get you green water..

@Torrey Green water is easy.. just get tank water and add lettuce leaves.. put that on the window sill and leave it about 3 or 4 days.. Infusoria which are the organisms in green water most fry feed on is also easy to make.. 1st way, the quick way.. Get a jar of aquarium water and add about 1 teaspoon of yeast powder to it.. I usually mix it with warm water first to dissolve it, then add it to the jar of water... Then put it somewhere warm.. most people say green water is best in cooler conditions but that's after you have it.. to get it started try it a bit warmer.. about the same temp as your aquarium. Im in Aus so for me its gong to be between 22 and 28 deg.. This should get you infrasoria in 2 or 3 days. 2nd way, takes a bit longer. get a bucket. i use a 20 litre/5 gallon one.. needs a lid... Next get a heap of java moss and stick that in there with enough water to cover it... then leave it awhile.. after a week or 2 you will have infrasoria in the water.. That can go for ages like that just continually producing inrasoria for years.

@Gardenman Yes thats probably true ... I have a small glass bottom tank Im keeping plants in pots in that needs re doing so Im thinking I might swap it into a flex that I removed the rear sump filter from and go with a foam base filter and see how that goes sitting the plants on that.. Problem where I am is Poret Foam is hard to get. I can get aquarium foam in sheets so ill try that out.

@gardenman Yep been done before years ago.. you actually dont need to use the UG filter under the foam.. the version Ive seen a video of they cut channels into the bottom of the foam to house air line under the foam .. There are a few good vedeos of one tank they use it on with Axolotls .. here is one for you.

Im a new member here and got onto this thread while doing a search for photo's of bottom of tank mattenfilters... I actually made one of these about 10-15 years ago for an Axolotyl tank.. Im actually a long time fan (over 30 years) of Undergravel filters and still use them in any of my tanks with gravel in them to this day.. I also swear by Mattenfilters as a concept when using glass bottomed tanks and had heard of them using bottomof the tank mattenfilters in some instances in germany.. They actully have been using them there for Axolotyls for a few years so Im maybe not the first to try it but I have never really seen much other than 1 paper on them using them for Axolotyl in Germany. That said, this is one of the first places Ive seen a discussion on using mattenfilters as a bottom filters for fish so great to see im not alone in my thinking.