Thanks for all the suggestions everyone. I unfortunately don't have any way to actually video the tank at night, however I will take some pics during my next shift. Here's a bit more info on the situation...
The filter system doesn't have a lot of suction that would suck the adult minnows into it. (Newborn guppys manage just fine.) And there are grates covering the intakes that have narrow (2mm) slits to allow water to pass through. Although the occasional dead fish gets caught on it, it's not strong enough to suck large healthy fish through it.
Another question would be how big are the loaches, and how small are the minnows?
The biggest fish in the tank are the blue redtail botia loaches. They are each approximately 3-4inches long and there are 6 in the tank. These are my primary suspects.
The minnows and danios are all approximately the same size, about 2 inches. Adults.
I know some can get to predatory sizes. I would definitely think you would see happy round bellies on those loaches though, after minnow feasting.
This is kinda what I was thinking too, and if you divide the numbers evenly it's only 4 minnows per large loach. I'd still think you'd see a bit of a belly on the loaches. Not only that but they happily devour their morning meals as well. Can't see them being that hungry after a night of feasting.
Do y'all have critterz gettin loose?
Altho the occasional animal gets out we haven't lost anything recently. Definitely nothing that would know how to get into the fish tanks and eat the fish... or want to.
Maybe the minnows were misplaced???
You are the VERY first on in the morning?
I'm the VERY last to leave, and the VERY first one to come in the next morning. They can't be getting misplaced cus there's no one to misplace them.
I don't think it's the khuli loach,
Agreed, they're smaller than the minnows are >_<
Google yo yo loach (or botia loach) eating other fish to see multiple accounts of fish murder.
I did so, but still doubtful. They are leaving the danio's alone and only eating the minnows that were of the same size. The yoyo's in the tank were about the same size as well.
Perhaps the danios were in the tank with the loaches all along so they do not equate them with food?
Nope, danios were added after the loaches, and were doing fine for about a week before any minnows were added.