Killifish are no different to other fish in regards their general care and keeping, so they do need a clean tank. However, you can have gravel, plants and ornaments in an aquarium and still have a clean tank.
Fish need a definite top and bottom in the aquarium. They need substrate so they know there is something there. And they need something on the back of the tank so they know something is there. Fish can't see glass and don't understand what it is. If people didn't know what glass was we would stress out too. There is nothing there and I can see through it but can't go through it. Ahhh, what is it? Fish are no different. They have never encountered glass in the wild and instinct tells them there is mud or gravel or sand at the bottom, and sky up top. And on the sides are muddy or sandy banks.
For years people have been saying Discus need to be kept in bare bottom glass tanks with nothing else in. Prior to the 1980s, Discus were kept in planted tanks with gravel on the bottom and they did fine. I have kept Discus in bare bottom tanks and in tanks with gravel and plants, and they do a lot better in tanks with a substrate and plants, and dither fish. In the 80s, Jack Wattley started keeping Discus in bare bottom glass tanks and he did exceptionally well with them. Because of that, all the Asian breeders started keeping them in bare bottom glass tanks and they did reasonably well. The fish had fewer health problems and were easier to treat. What wasn't known is most of the problems associated with Discus at the time (and to present day), is caused by contaminated food being fed to the fish. The Asian breeders feed the fish on various organisms that are grown in sewerage farms.
Wild Discus don't live in a pristine environment and neither do killifish. They live in creeks and rivers with mud on the bottom and vegetation growing in and out of the water. Some killifish (Nothobranchius) actually lay their eggs in mud and the eggs sit dormant in the dry mud waiting for the next rain. When it rains, the eggs hatch out and the baby fish grow rapidly and produce eggs of their own.
Nothobranchius eggs are regularly packed in damp peat moss, which is put in a plastic bag and shipped all around the world. The eggs remain viable but dormant in the peat moss for months.
If you want healthy fish, do regular water changes and gravel cleaning, as well as filter maintenance. If you gravel clean a tank containing killifish or rainbowfish, suck the gunk out into a clean white bucket. Leave the water to sit and then look for eggs on the bottom of the bucket.
I used to gravel clean the tanks into a bucket and leave it there for a week and simply scoop the fry off the surface as they hatched, and move them into a rearing tank.
