Hi there,
I'm sorry for the loss of your frog.
Curious, what type of treefrog do you own?
There is absolutely nothing bad/wrong with reptivite in use with tree frogs. I used it in the past for a long while with no problems what-so-ever. Reptivite is a muti-vitamine only, not a calcium supplement. Only dust feeders once per week with the reptivite and separately dust with a calcium and d3 powder every other day.
What was the reason for mixing it with water? I've never heard of that before, and have only dusted feeders with the powder.
Perhaps the mealworms drowned? Or, I know crickets intake oxygen through their exo-skeletons, maybe mealworms do also? If the water and reptivite solution completely covered the exo-skeleton, perhaps this would explain the death of the mealworms? Just a guess....
Mealworms aren't a recomended food sourse for treefrogs anyhow, the chitin is difficult to digest and they have been known to be the cause of impaction, just fyi.
I have no idea what would have caused the death in your frog, but I doubt it was the reptivite.
I do not use reptivite anymore, I use Rep-Cal Herptivite once per week, and Rep-Cal phosphorus free calcium with d3 every other feeding (about 2-3 times/week).
I have also used Repashy Calcium Plus for dusting feeders, this product is an all in one cricket balancer. I liked it and would use it again...
Great that you were able to clean out the enclosure so quickly, and that your current frogs appear healthy. You should still keep a close eye on them.

If you can answer these questions (which are found in the sticky, Trouble in the Enclsure), someone may be able to help with what may have actually happened to your treefrog:

1-size of enclosure
2-# on inhabitants - specifically ( if there is another frog ---size differences ?)
3-humidity
4-temp
5-water - type - for both misting and soaking dish
6-materials used for substrate
7-enclosure set up i.e. plants( live or artificial) wood, bark etc -how were things prepared prior to being put into the viv
8-main food source
9-vitamins and calcium ? ( how often )
10-lighting
11-what is, specifically, being used to maintain the temperature of the enclosure
12-when is the last time he ate
13-have you found poop lately
14-a pic would be great ( including the frog ) any little cell phone pic is fine
15- how old is the frog
16-how long have you owned him
17- is the frog wild caught or captive bred
18- frog food- how often and if its diverse what other feeders are used as treats
19- about how often the frog is handled
20-is the enclosure is kept in a high or low traffic area