I've had my kuhlis for six years, and I just love them. In a tank devoid of any fish to harass them they should be active all the time. (I keep mine as pretty much a species tank now because of trial and error with other fish, though there's a betta in with them now.) They are smaller and look like worms so larger fish (like Opaline Gouramis) will attack them. They also are not good with any other bottom dwellers as they have no respect for personal space. They also can't seem to tell that other fish are not like them and will try to school with anything that moves, so they can actually cause stress to other fish who aren't comfortable with a different species banging into them on occasion. When you go to introduce more kuhlis into the tank they can seem to sense the new guy while he's still in the bag and will swim up and swarm the bag until he's released - it's awesome to watch.
Mine are never hiding. Larger groups are a must and although three is a bare minimum they will act more normally and be more active in groups of more than seven. They need good cover in the forms of caves or driftwood or large pebbles creating hiding spaces, and the more hiding places available they more they'll be out since they'll feel safer. They do not like direct light and will stick to the shadows. (I use floating plants for this.)
I've used both gravel and sand, and both have their perks. Gravel will give them a darker and prettier coloration and give them a chance to hunt for food which keeps them occupied. However, it's rough on them and they can't mold the tank the way they want it. Sand will allow them to sift for food (taking mouthfuls and passing the sand through their gills like Gobies). The tiny pits they make are so cool to see in the morning since they are everywhere! They'll also push the sand out from under objects and make their own caves, though nothing that will destroy a tank. Normal sand will wash out their color making them very pale. I just tried mixing the two substrates last month and the kuhlis I think are finally in heaven.
They can sift or hunt, their color is great, and they still have their caves. (Still working out the best way to keep the pebbles and the sand looking nice, but the fish are happy so I'm happy.)
Personally, I'm hoping to upgrade to a larger tank so they can have more room to swim. They've been in a 20 gallon for the past five years and I've always felt they needed more space but I never had the funds. They are VERY fast swimmers and when they spook (every time the lights are turned on in the morning) they slam into the glass which is upsetting to watch. They'll also spend hours lapping the tank in circles or doing loop-de-loops vertically up the glass. They are very active and need more space to move then their small size would indicate.
Frozen proteins of all kinds, from shrimp to bloodworms, will elicit insane feeding frenzies. (Frozen simply because it sinks quicker than freeze dried.) They are slow feeders and do have problems reaching food before other fish get it. They're blind as bats and can't smell well in a small tank with recirculated water so they find food by bumping into it and they tend to loose it again between bites.
If you have the space and a community of peaceful small fish then they are really cool additions. They are supposedly very long lived as well (I've read reports of over 23 years), so they should be with you for a long time!
