Describe one way that freshwater animals can compensate for ions lost during excretion of excess water

They replace the lost ions by active uptake through active transport of ions from the surrounding waters.  This is exhibited in freshwater fishes that actively take in these ions through the gills.  Most of the ions involved in osmoregulation in freshwater are those of Na+ and Cl-.  

do you have a picture?

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%