Im concerned about a frog and my pond and dont know what if anything is wrong.

This morning I had two big dead like leapord? slugs floating in it (but these seem to be aquatic and leapord slugs arent supposed to be so I dont know exactly what kind of slug they are but Ive been watching them moving around in the pond or up pond sides for at least a week). I cant figure out why a big one of those and a littler one, suddenly died in the pond today.

Then this evening, one of my new baby frogs (from tabpoles Ive had in the pond since last xmas) was sitting on the rock in the pond, rubbing its eyes over and over and then with its back legs, rubbing its stomach (as if it has belly ache) and it was also then opening and shutting its mouth over and over (maybe gasping a little). Making me wonder if it is soon to die. When I went to catch thou to isolate it to further keep eye on it, it jumped back into the water so I dont know where its gone now thou may find it later (unless its died now at pond bottom). The baby frog had been growing well past few days and had possibly doubled its size but now this.

I top up the pond regularly and I also water plants around it from the pond water so there shouldnt be a build up of any bad stuff in it. I also dechlorinate any water I add to the pond before adding it in. Ive had this bathtub pond going on since start of the year and till now have had no issues with it at all, all the frogs coming out have been healthy (thou disappeared soon after).

I did add in a dead or nearly completely dead and dried out hibiscus branch a few days ago into the pond, to give the latest batch of new frogs (I hadnt had any of the tabpoles turning to frogs since the beginning of the year) a place to sit before they climb out as they arent treefrogs. I dont think hibiscus is poisonous? as I know we can eat the flowers of it. I've also had a hibiscus branch in the pond before with no issues at all.

Any ideas at all what could be wrong?

I do have a lot of natural pond life there of late.. maybe it ate somethign too big for it? I currently have like those things like dragon fly nymph climbing out of the water in the past few days..maybe its choking on one of those?? (something seemed to be moving around in its stomach). I also have massive worms in the pond.. and snails)