Oh - using ethanol to dose prazi is REALLY risky! You need to use the absolute least amount possible and aerate the heck out of the tank. What happens is that heterotrophic bacteria act on both the ethanol and the prazi, using oxygen in the process. If you use just a touch too much alcohol, the tank clouds up and the fish can asphyxiate.
Prazipro uses a glycol as a solvent. It has the same issue, but to a lesser degree. I've never found any shelf life issues with it, but I've also never had a bottle around longer than a year.
For my main systems, I use bulk praziquantel that I express through a fine mesh brine shrimp net, making a suspension.
The biggest issue with "failed" prazi treatments is due to two factors: 1) Neobenedenia eggs are not killed by prazi and they can take up to 30 days to hatch. 2) the bacteria I mentioned above get really good at consuming prazi. After a couple of treatments, the population is high enough to strip the prazi out of the water before it has time to work.
Jay