Hi,
So I have three main problems, just reply to whatever you can, I would really appreciate your help!

1. Death diagnosis?
I got two white's tree frogs about 6 months ago (from Petco, unfortunately). My parents set up their vivarium while I was in a different state, and they purchased a waterfall (which I was ecstatic about), but it requires about 4-5 inches of water at the bottom of the tank to work. I did a complete water change and scrub-down about 2 weeks ago, and this morning I woke up to find my littlest frog floating in the water. I fished him out and determined he died I'm just wondering if it's possible for tree frogs to drown? I didn't see anything else wrong with him and he was fine when he was alive (lively, pooping, eating, etc.). I just want to make sure that my other frog isn't in danger of drowning.

2. Frog depression?
Is there a risk of my other frog becoming depressed? I know this can happen in pets like cats but I'm not sure if frogs have complex enough brains to experience depression.

3. New vivarium/terrarium.
I am getting a new enclosure this weekend ( 45 x 45 x 60 cm / 18” x 18” x 24” (WxDxH) at Exo Terra : Natural Terrarium Small / Advanced Reptile Habitat), and I was planning on getting a new companion for my other frog. However, the frogs at Petsmart are twice her size, and the frogs at Petco are half her size (TINY.) There is no reptile store here, I would have to drive an hour to get there and I don't even know if they have any. My question with that is, would it be better to get the smaller or larger frog, and how do I determine if the new frog I choose is healthy? Should it be extremely lively, or fatter/bigger than the others?
Also, if you have any suggestions on things to put inside the terrarium (other than substrate and other basic things) I would love to hear them! I am switching from a horizontal terrarium to the new taller one (linked above). The guy at the reptile store (I love that place) said the terrarium would be big enough to house two adult frogs comfortably.

Thanks!