Curious as to whether the white spots are more in the rays of the fins. Os are notorious for getting calcium deposits where they damage fin rays. Other than that Gerry has you covered.
Growth rate on a healthy O can run .5" to 1" a month...so any tank appears to shrink quick. 48g (180L),with them being around 5",means you should start looking for an upgrade now.Especially having two.
Filtration will need to be a healthy flow rate too. Depending on the filter type used it should be 10X tank volume hourly with hang on powerfilters (HoB),5X tank volume with canisters,or 7.5X with a combination of both.Those are minimum values...more always better.
Just think about it.