I'm sorry for your loss. Let me see if I can help.
Half water, half moss sounds very wet. Combine this with the clue that each time it has been their feet and that sounds like mold, fungus, and rot problems that they cannot get away from. Tiny little scratch and they cannot escape the wet conditions.
75 sounds big for 8 frogs (or even just the 6 remaining) but if it's half water you are cut down to 24 by 18 inches. That's not big enough for 6 frogs, let alone 8. If you have a background that further cuts down of floor space. Keep in mind the ponds are ONLY for you. Your frogs would much prefer more land space. Further, you will typically NOT find much moss in their natural environments. Instead, you will find leaf litter.
My suggestions:
1) If you feel you have to have a pond, reduce it's size by 75 %. Fill it in with more land space.
2) Do you have a proper false bottom to allow good drainage for the (too wet) conditions in your viv? It needs one. Or a nice thick layer of LECA clay balls.
3) Rip out 90% of that moss. A small patch here and there is OK. If you want, save some to mount on your background. Your frogs don't want moss. They want leaf litter. Lots of it. Fill in all the rest of your ground space with leaf litter. 2 inches thick is what I strive for. Your frogs will love it. They will hunt in it, play in it, sometimes hide in it. They will learn that cover is only a hop away and will become more bold for you. Your microfauna (if you don't have any, get some) will reproduce and thrive in it. Microfauna is springtails, Isopods, and other tasty snacks.
You've tried pretty (moss everywhere and half water), and you've seen how much your frogs liked that. Now try functional and see how much your frogs will love it!