Help! Want tank to go to the next level.

I need some suggestions/help with how to take my tank to the next level. I’m a science teacher and this tank lives in my classroom

Here’s what I’ve got:

29 gallon tank, Tetra 10-30i filter, Under-gravel filter, Rocks and wood for decoration, Bubbling decorations, Plant substrate (not gravel), Various plants (see photos).

2 nerite snails, 2 otocinclus, 2 ancient zebra danios, 3 fancy guppies (1 m, 2 f), 1 green Cory catfish (with a messed up tail), 6-ish blue wag platies (they keep having babies)

Temp is 68 F. pH is 8.5 (I have a pH prob I use to check, so this is unfortunately very accurate). Very hard water (see photo)

Problems: Can’t keep plants alive, Algae, Fish are disappearing, yes there’s a chance kids are screwing with me, but I really don’t think so. I don’t find their dead bodies anywhere in the tank and didn’t think my other fish could eat them that fast. I had some black mollies and sunburst platies that just left the chat. Yet my blue wag platies and guppies are all gravid right now and are always popping out babies.

From the little research I’ve done on this subreddit it looks like I need to add CO2 to keeps plants alive $$$$. And there’s not much I can do about the hard water.

What advice can you give me to keep plants and fish alive? What species of fish can I put in here that will do best in these conditions? Do I need a new filtration system? I want this tank to be awesome and it’s pretty mediocre right now.