Hey there. Welcome to the joy of keeping red eyes. I found them a bit tricky at first as well, and know how bad it sucks when you find one dead despite your best efforts. Once you get your enclosure set up properly it gets a lot easier. There's a good care sheet on this forum that's a good place to start.
Frog Forum - Red-Eyed Leaf Frog/Red Eyed Tree Frog Care - Agalychnis callidryas
Straight off, you said you bought another tree frog from a pet store. Is it another red eyed tree frog? You should never keep multiple species in an enclosure. Some are toxic to each other, and cannot be kept together. I've heard of people keeping very similar species together without problems, but generally it's a bad idea. Also, if the other animal is sick it can spread the illness to your other animal. Especially true with wild caught specimens.
Temperature and humidity is key with Red eyes. They like it pretty moist and hot. Day: 26-28 °C (78-82 °F); Night: 22-24 °C (71-75 °F), with humidity around 80%.
This can be tricky to maintain in enclosures, especially for red eyes as they require a lot of air circulation, and need screen lids. Be sure to spray them down well at night. I use distilled water for misting as it doesn't leave deposits on the glass. Monitor them throughout the day to spot spray as necessary.
Buy another hydrometer if you think yours is faulty. I find the digital ones to be a bit more accurate.
They require minerals from water to stay healthy, so give them a bowl with either spring water, or treated tap water.Make sure to change it out often, preferably daily. They like to use it as a toilet as well.
Feed them crickets no larger than the space between their eyes. Dust them with a calcium cricket powder supplement, such as ReptoCal. Crickets are not a complete meal sadly, and they can get malnutrition from a diet of just crickets, that can lead to bone problems and prolapses.
They tend to get stressed pretty easily, handle them as little as possible. It might take a while to get used to the new enclosure, so they might sleep a little more than usual. Keep in mind they sleep around 17 hrs a day. They sleep a lot, and usually wake up for just a few hours in the middle of the night.
Watch for red flags as well. Especially if the frogs are sleeping on the ground. This is a sign of a sick or dehydrated frog.
Spraying them with water while misting down the enclosure is fine, but I wouldn't spray them too directly. Having a little eco-earth stuck to their skin is not really a problem to them.
Hope I answered some questions, sorry if I missed one.