Nitrate levels high

Question
QUESTION: First of all my setup & bio-load:
125 gallon freshwater tank with wet / dry sump rated at 300 gallons.
In the sump I have coarse filtration, then 100 micron filter flowing over bio balls then passing through a polishing block before water is returned to the tank.
Bio-load two nearly mature Oscars purchased 4/11 One Tiger oscar aprox 10" (Grouper) and the other a albino tiger aprox. 9" (Flounder).
The ammonia levels stay fine, very low. However I cannot seem to get the nitrate levels to drop below 40-80ppm.
The water is monitored with API test kits, not the strip types.
I thoroughly clean the gravel removing 15 to 20 gallons per week and partial water changes (60+ gallons) at least monthly.
I feed them about 30 feeder fish weekly and small amounts of beef heart between feedings. There does not seem to be an excessive amount of waste when I clean the tank just cloudy material when I sweep the gravel.
What do you suggest? Additional nitrate reducing bio pads?
Less feeding?

ANSWER: Hi Zack,
Thanks for all the information about your tank setup, fish, and care routine!

I've found with Oscar tanks, that it is a challenge to keep the nitrate levels low. They are a high-bioload fish. In that they produce far more waste than other species. Your Oscars are also fed well, which causes pollution levels to rise even more rapid than if they were fed on a diet of say just pellets.

There's nothing wrong with offering feeder fish. They can be diseased unless you quarantine them. I prefer if you do offer live feeder fish, to keep them in a separate tank, feed them well and change their water at least on a daily basis so they are at their healthiest when you feed them to your Oscars. Feeder fish in stores are often in very poor condition, usually aren't fed, and can carry illnesses.

Also for the best health, I would recommend supplementing Grouper and Flounder's diet with a high-quality Oscar pellet food.

The best way to determine how often and what percentage to do water changes is by monitoring your nitrate level. Since it is high, I would try changing out the amount you usually do once a week, but do that twice a week and see how the nitrate levels compare.

If they are still high, you may have to increase the percentage you take out each time. Its all a matter of experimenting. Remember the solution to pollution is dilution!

Best of luck with your aquatic pets and I hope this helps!
Susan~

---------- FOLLOW-UP ----------

QUESTION: The fish pellets would be except for one thing. They will not eat them. I started them on pellets but I have spoiled them with feeders, beef heart, crickets, and frozen brine shrimp. I use Microbe lift Nite-outII and Microbe lift Special blend in proper maintenance doses, 25 mil & 30 mil respectively. And I've added moss balls since I've read live plant life helps reduce the bad stuff. Any ideas on filter media?
Thanks,
Zack

Answer
Ohh yes! Oscars are easily spoiled! You can help make up for the lack of pellets in the diet by providing a variety of meaty foods like crickets, shrimp (the salad shrimp sold for people even), earthworms, healthy feeders, small portions of beefheart and so on.

Good idea with adding the plant life for nitrate reduction. But unless you have a vast array of plants almost like a real garden it wont be enough. There really is no substitute for water changes. The best breeders and experts will tell you.  

The current filter media you have actually sounds just great for your setup. Remember, filtration can only deal with so much pollution. It cant remove nitrates in most cases and even filter media designated for removing nitrates is quickly exhausted and is rendered ineffective.

Best of luck and i hope this helps!
Susan~