Chlorine is a fairly volatile substance, it will dissipate to the atmosphere or bind to other substances if you let the water sit for a couple of days. Heating it will speed the process. Chloromines are more stable chlorine compounds that will not dissipate so easily. For the health of your frog, just get some water treatment drops. I have 4 Pac-Man and 2 tree frogs, a large bottle of reptisafe cost about $9 and lasts me around 6 months, and I’m super liberal with it’s use. Small price to pay for some safety for your frog.