i have never heard of a more overstocked tank than that. you really should rehome some of your fish-i know you love them but some of those fish are not really meant for aquaria-like the pacu's- let alone 3 of them.
since money is no option, you need to buy a freshwater test kit. check your ammonia, nitrites and nitrates. im betting you have done so much waterchanges you are now cycling a VERY overstocked tank and have ammonia present-thats why they are gasping. with ammonia poisoning you will also see redness near the base of the fins and gills.
i would suggest you rehome alot of your stock as your tank is just too small, and they will probably die before you can get a bigger tank since they are in distress already.