I am a new member, I was looking for people to help me deal with the situation in my tank

In short, I got a 54 liters tank and last week I was looking for a couple of small corys to start it off. As the tank is so small I was hoping I'd find some habrosus or hastatus. In one shop they had 2 tiny corys in a tank with a lot of shrimps, a large betta and an agitated bunch of endlers. The catfish didn't move and looked very unhappy. They were labeled as c. habrosus. I went to another shop where they had another couple of the same small catfish, this time labeled as c. hastatus, in a tank with c. pigmaeus and lots of tetras chasing them around for fun. So I got the two little ones and went to the other shop and got the other two. I accommodated them into the tank and then did a little research on the web only to find out that not only they weren't either hastatus or habrosus but they weren't corydoras, they were aspidoras pauciradiatus.
I read all that I could find online, so I replaced half of the gravel (the front half, the back will be planted) with sand, and I know my pH is probably too high to reproduce them (7.5, tap water is 8 so the water changes raise the pH instead of dropping it).
They are active sometimes, at other times they just hide in the plant and in the decoration. Two of them are more active (swimming up and down the aquarium sides), they rarely hang out for a few minutes all four. I can't really tell whether they are happy or unhappy.
But my big problem is: according to stuff I found on the web, they are voracious eaters ... I got them tetra tabimin tablets but everyday I have to siphon the tablet out of the tank a few hours after I added it, as they are not interested. Sometimes they go near and sit immobile but I can't see if they taste some of the tablet powder, as they won't stay there if they see me. They "kiss" the decoration and the sides of the technical parts in the tank (internal filter, heater), but I don't think they find any goodies there as the tank is so fresh (two weeks now). I don't know what to do. I normally avoid no name tablets and I don't think getting other tablets will work. I will go tomorrow and get them some frozen brine shrimp but it may be too large for their little mouths.
Does anyone else have experience with this problem? Will they eventually adapt to it? Or are they just likely to starve to death with the tablet in the tank? As I write there is one sitting immobile a few centimeters away from the tablet, maybe it's eating... but they don't stay there long. Is it possible that they eat SO SMALL that they are unable to finish off a quarter of a tablet in a day? Uhh...

Thanks if anyone has had the patience to read all this
