Clown killies keep vanishing

[new_friends]

So, first and foremost, I know killies are jumpers and the tank has a lid that fits tightly and a water level a couple inches below the lid. That said - I keep literally losing clown killies. I'm not finding them on the table or floor, and I've moved everything and looked, and I cannot find any. I really don't think they're jumping. The filter is a sponge filter, so they're not going in there. I have never seen a carcass, though they're small enough I guess the snails or their fellow killies might be eating them quickly. But what the heck? I had 6, all fat and sassy and eating well. One male is larger and more brightly colored, and will sometimes run the others off, but I put food in 6 different spots to try to ensure everyone eats. Slowly they've just vanished without a trace. Last week I was down to the big male, a slightly smaller male, and a female... and this week the female is gone. I don't want to replace them unless I can figure out what is going on. I even had fry at one point who got up to about the size of a grain of rice and could be seen eating well. Mostly they eat vinegar eels supplemented with "fry crack" 

Water should be good; room temp (between 70-73F), TDS around 250, pH 6.6-6.8, kH is about 40ppm, gH about 300ppm, no chlorine. 0/0/trace ammonia/nitrite/nitrate. Anubias, java fern, a nice cover of dwarf water lettuce with trailing roots for them to hang out in. I've got 6 other tanks/ponds and this isn't happening in any of the others so I don't think I'm like, inadvertently poisoning them with residues or my aged tap water (also treated with Prime). 

[MrGibson]

What kind of snails and how many? A big group of ramshorns woils probably leave both behind in a matter of hours with something as small as a clown killi. 
 

As for why you’re losing them nothing immediately jumps out at me. How big is the tank and how long has it been running? How long have you had the killies and did they come from a south you trust?

You mentioned a sponge filter, how big is the hole for the air line? I’ve not personally kept clown killies but I wouldn’t be terribly surprised to hear they can find their way through a 1/8” gap with some determination. 

[beastie]

I am super curious about this. Maybe a tank picture? I have them too, and while they chase violently, especially the males, they are otherwise very mellow. Maybe the lack of visual barriers or hiding places? 

I have a gap on the lid because of light stand and I have never ever had any one of them jump, despite having a 1cm gap. They dont like flow, do you limit the sponge?

[Schuyler]

You say the lid is tight but the first thing I would do would be to put plastic wrap over any gaps and then see if any more disappear.

Just the other day I was working on a tank when suddenly this tiny grasshopper landed on the lid. Which was weird because it was in the middle of my garage up on a shelf. Then I realized it wasn't a grasshopper, it was a killi fry. It jumped again and it got like 4-6" in to the air. 

If one disappears after that you know it's something else. But a killi disappearing without a trace my first thought would be jumping. The next guess would be they died and got eaten by snails or something.

[new_friends]

So sorry - I didn't get notifications that I had replies, thanks gmail. Anyway!

I ended up putting them in a different tank with a lid and some duct tape (folded over in the exposed part so it's not sticky to them) for now. I do limit the flow - I have a regulator on all my sponge filters and had that one set to a bubble every second or two. Pretty still except in the very corner where it sits. The new tank is a 20 long and has a bit more flow on the filter side but lots of still spaces in floating plant roots as well.

The snails are ramshorns, some of which are quite large (maybe 1" diameter). 

That tank was running for... hm. I think 8 months? The tank I moved them into is probably more like 6-7 months. So far they are doing well in there with a few small yoyo loach juveniles at the bottom. I can get a better view on them as well because the tank is up higher, and they hang out at the surface under the plants. So I'm hoping that either everything is OK, or if whatever does go wrong, I will see it. I'm going to set up a little webcam to try to look in after dark as well.