This system is in my friend's bedroom, and he asked me to make the drain quieter. Here's what I did, and it worked really well. After building this muffler, you can't even tell when the water is draining unless you're watching the water level in the fish tank.
This was the sketch of my idea. The drain pipe would point down into the fish tank, but would not be submerged in the water. A larger diameter pipe would sit around the drain pipe, with the bottom end submerged. The larger pipe would be held onto the drain pipe with a wooden dowel through both pipes. The larger pipe will muffle the sound of the splashing water. To reduce the noise further, you could plug the top of the larger diameter pipe with cloth, foam, saran wrap, etc.
I ended up using saran wrap, and the larger diameter pvc pipe wanted to float, so it started leaning to the side. This may help the muffler, because the water runs down the side of the pvc pipe rather than splashing into the tank. I'm planning on replacing the saran wrap with something better looking, but I want to test some options.
Our tilapia Zeus seems to miss the splashing water, though. He used to like being splashed with the draining water. Oh well, it needed to be quieter.
And the garden is looking good. The growth has been slow, because the water was pretty dirty. I did a partial water change, and cleaned the filter and plumbing. I hope that helps the plants a bit. After the water change, the nitrates are still high, and the pH is about 7.5. I added a little pH down to get the pH closer to 6.8.