I will leave the real answer to one of our other resident FW pros. IMO though it sounds like a good approach. Not a permanent one by far, but a good start.
What filters do you use in your tank?
My suggestions would be:
- 50% water change.
- Thoroughly vacuum out the the substrate to remove all the waste you can.
- Buy a replacement filter and stick it in the tank for a week or so before changing out the filter so that you preserve at least some of the bacteria.
- Wait a day and change out another 25% of the water while vacuuming out the substrate again.
- Re-evaluate your feeding schedule.
- Use some of the bacterial supplement mentioned to further seed the new filter media.
Remember these are just suggestions from my little experience.
I would point out that 3 Oscars are going to produce a lot of waste and you are going to have to stay on top of your regular maintenance of changing water out if you hope to keep things under control.
Feeding live foods is not ideal for preserving water conditions. Its great fun to watch, but doing so only increases the waste production over normal food.
HTH,
Aaron
Found water cloudy today and said maybe i'll take a sample to my local pet store. So i went and found out that my water levels were PH=7.6 Ammonia=0 ppm Nitrite=1.0 ppm Nitrate=40 ppm. I was told to do a 50% change today and replace my filter carbon and add some good bacteria (micro-lift, special blend). Before i do anything do anyone know if this is the right direction to go? Just for information this tank been up and running for 5 weeks, and water a 20% water change is done every week. Tank has 3 oscars and one pleco, feed once aday. 
