not sure what type of tank or lid ya got but i have perfecto tanks and lids...there is very few openings in these brands ..even the knock outs for equip can be as small as needed,, my water evaporates too i add about 1/2" once a week, the perfecto lids are great and most of the water condensates on lid and returns in it........
i also use 2 bio wheel filters on my 55 peingun 330's and where the area is for the knock outs for filter bio wheel i cover with the plastic the pkg cover from the plastic plants i got , FITS PERFECT!!
think some of the ones i used were from the tall 20" plants, this prevents + controls evaporation + fish jumpimg out too..
i think i bought hagen plantastic plants, the are pkged in a nice clear firm style plastic material that works well for this aplication..
BUT EVAPORATION IS NORMAL........only a tight fitting cover prevents abnormal amts..