If you want to raise the pH of your tank then aragonite sand or crushed coral works wonders. I use it to raise the pH in my goldfish tank.
That said, back to your original question. Freshwater refugiums are few and far between, but I've done some research into them because I own goldfish which aren't compatible with most aquarium plants. Most of the refugiums I saw were either on goldfish or cichlid tanks. There are two routes you could go: under the tank and submerged; above the tank and immersed. If you have a spare 10 or 20 gal and feel like doing some plumbing then under the tank might be the way to go; if not then a hang on the back with immersed plants sounds like the way to go. The idea behind those is the plant leaves are out of the water and the roots are in the water. The roots will pull nutrients from the water and the leaves will get CO2 from the air just like floating plants in the tank.
I want to try something like this on my goldfish tank. I'll have some immersed houseplants in the HOB on the back of my tank. I'll put a desklamp with a daylight spectrum CFL so I am not limited by ambient light. I want fast growth as the goldfish tank can get high nitrates from time to time.
I was going to link you the website I found detailing a goldfish immersed refugium, but it appears the website has been taken down.