Tiger Barbs not eating

[newaquariumguy]

Hi everybody.  I have 9 Tiger Barbs, one RTS, 4 Bronze Cory', and 7 assorted Platies in a 60 gallon.  Plenty of filtration with a Fluval 305 and a Fluval 306 canister filter; decent (?) heater which is a Fluval E 300.  It has been cycled for about 3 months now and using the API liquid test kit, I can continually get readings of ammonia  0, nitrites  0, and nitrates 20.  I change about 25 - 30% water, weekly.  I have been testing it and after leaving it for 10 days, the ammonia creeps up to .25 while the nitrates will go almost to 30. I have been told that that ammonia reading might actually be ammonium, due to the use of Prime.  Although being new to this, I would call the tank medium planted--maybe a little less.  I use Flourish Excel and root tabs.

The fish all seem to be happy and healthy and I am amazed at how fast they actually grew.  I have learned that I was previously over feeding and have got that under control now for over a month.  I feed NLS Community pellets along with a pinch of flakes twice a day.  I drop some sinking shrimp pellets or a wafer for the bottom guys.   Usually one day a week, they are not fed.  I have tried peas (they all would rather starve to death than eat them) and cucumber which is not going over great with them but they will eat it if not fed for 2 days.   Twice a week I cut rations to about 1/3 and give them either dehydrated brine shrimp, tubifex worms, or blood worms.  They all go crazy for that.

Now the Tigers do not eat anything except the times when I give them the treats.  Have I spoiled them?  Am I getting worried about nothing?  Will they eat if they are hungry?  Do they eat less now that they are finished growing?   This not eating thing has been going on for about 2 weeks now.  

Any suggestions will be appreciated.  Thanks