Persistent HIGH NITRATES

[jpeeples001]

I have a 115 Dutch Aquarium System. I boughtit used in September and had great test results for nearly three months. I had a satble supply of 35 inches worth of fish, approximately 15.
In November I added two new DAS filters that I bought thru my local store, in Pittsburgh. Previously I used a cottonly material, wrapped around carbon that came with the orgiinal tank but was rather old. In short, since November the nitrates have been 801-20 ppm and off the scale. I've done nearly bi-weekly water changes, of 20-30 gallons at a time. My tank fionally crashed the day after I left for a ten day vacation over CHristmas. The kid fed the zebra eel and ridgeback eel two 75 count deli fresh shrimps and a teaspoon of frozen brine, as I'd instructed and the next day all fish were dead except the sponge starfish and the two eels and the biggest trigger. I have several corals that have done well, and are still okay. We lost about 14 fish though, and eventually the Zebra eel died right after CHRSITMAS. I have no idea what went wrong, possibly the two cukes died and polluted the tank rapidly. I have done three water changes in the last week: 15 gals, 20 gals, 35 gals and my nitrates are 80-120 ppm still. DOES ANYONE KNOW HOW TO GET RISD OF NITRATES SHORT OF STARTING ALL OVER AGAIN? I'm worried about dumping all the 115 gals in that I may lose the healthy soft corals, the starfish and the two last eems and Picasso Tripger that remain. I'm rather fond of the bunch. This is a new Zebra my husband bought for our Jan 1 anniversary to console me. HELP!!!

 

[polarpooch]

Poor girl. What a disaster!
Don't know if this will work for you, but I had a problem a while back with wildly high nitrates in my moray tank. After several water changes and gravel cleanings, they barely budged. My eels were laboring and gasping. So I got some Kent Marine Nitrate Sponge and added it to my power filter. On the last (massive) water change, the nitrates were at acceptable levels, and have remained so ever since.
I also checked with an eel expert, who told me that I fed my morays too often (used to feed every 2-3 days), and that likely contributed greatly to the nitrate issue. Lowered my temp 2 degrees to reduce their appetite, and have been feeding them once every week to week and a half. I do a 20 percent water change and gravel cleaning the day after I feed them now. Nitrates have remained stable.
You might want to change most of the water in your tank, and make sure your filtration is working properly, too. Remove the new eel and inverts to buckets and get at it one more time. Vacuum that gravel good, too!
Good luck with the new Zebra. Sorry about your other losses