First water change in over a year?! :o Even with freshwater tanks you need to do water changes every so often even if you have a good filtration system going. You only have two fish in the tank which is somewhat of a light load for the tank, but over time the excess nutrients (uneaten food and fish wastes) continue to build up. If you don't have any live rock, a deep sand bed, or refugium on the tank then the only way to get rid of these is through water changes. I would recommend more frequent water changes and keep an eye on the levels before adding anything. As Josh asked, what else do you have in the tank? Live rock acts as an excellent filter system. If you add some to your tank, you need to make sure it is fully cured so you do not force your tank to cycle again and stress your fish.

Also, the white film you found when mixing the salt water happens sometimes when there is not enough circulation. The air stones probably didn't do much for circulation and just added a lot of oxygen to the water instead. Try mixing a smaller amount of water (changing 20 out of 42 gallons is too much at one time anyways) and hopefully your Rio will handle it better. If not, just get a bigger pump or add another power head to help the one you have.

Good luck and as Josh mentioned, welcome to MAAST and don't hesitate to ask any questions you may have.