Yes, if you are using tap water and adding dechlorinators or water conditioner, then your skimmer will definitely overflow. You need to use RODI water. RODI will physically remove all bad particles and elements from the water and leave you with good water for your tank that does not need to be treated. All you need is RODI water and salt.
If you were adding dechlorinators and water conditioners, then you need to stop adding them to the water and perform water changes without them. Also, you need to run activated carbon to remove the left over chemicals that water changes cannot remove. Once all the chemicals have been removed from the water you will see that the skimmer will no longer overflow.
Also, please be sure that the skimmer is only sitting in 5.5"-5.75" of water. That is the measurement from the bottom of the skimmer to the water surface line outside the skimmer. Anything above that and your skimmer will overflow. Anything below that and the skimmer is too shallow and will not collect.
Any RODI on the market would work. We do have one ourselves that will be a very good unit with a lot of features that are not available on any of the current units out there but the unit will not be available for sale until September.
Let me know if there is anything else I can help you with.