ammonia won't go down

We have a 75 gal tropical aquarium. It's been set up for about a year. About 3 weeks ago our ammonia spiked up to stratospheric levels-the test tube reaches 8ppm in less than a minute. We've changed the water multiple times, put in ammo-lock, greatly reduced the amount we feed our fish, replaced the regular carbon in our filter with something that's supposed to bind the ammonia twice and poured in a bottle of stuff the guy at the fish store swears by. We've lost about 3/4 of our fish-we're down to a giant plecostamus, a reed fish, a kissing fish, an opal gourami and one other dude.
Our last full set of readings were...
pH-6.0
Nitrites-5.0
Nitrates-40
Ammonia-8.0

Please Help-we've already lost some of our coolest fish!
Thank you...
Rachel

A few weeks ago, somebody wrote in with a similar problem. Turns out, one of the children in the family thought the fish were starving because they always acted hungry. Without telling anybody, she was dumping in huge amounts of fish food. Is it possible something like that happened to your aquarium, or is it possible that you recently medicated the aquarium to deal with parasites or a disease? Medications can kill the good bacteria that convert ammonia to plant food. That, like over feeding, will stop or unbalance the nitrogen cycle and result in high levels of dangerous ammonia. New types of fish food or new decorative items not intended for aquarium use could also upset things. I like to feed frozen brine shrimp and similar products, but I make sure they are well thawed, and every so often I give them a good smell to make sure nothing is rotten. It happens.

About the only way to deal with a broken nitrogen cycle is daily water changes of 25 percent or so. Unless there is a problem with your tap water, the ammonia and nitrites should come down in about a week, maybe longer. As part of the water change, you should syphon most of the gunk from the aquarium gravel. When you do the partial water changes, condition the new water with something like Stress Coat or Amquel Plus. Live plants will help absorb some of the excess nutrients and thus keep the nitrogen cycle in balance. An additional air stone will also help by putting more oxygen into the water. I would avoid most of the chemical fixes, because they are easy to over do, and they can change aquarium chemistry too fast. Rapid changes in water parameters can kill fish and, for all i know, beneficial bacteria as well.

A 75 gallon aquarium is pretty tolerant of mistakes, so the problem could be something like long term over feeding. Once you find and solve the immediate problem, the challenges will be maintenance, biological load, and species compatability.

I hope some of the above ideas have helped. Please keep us posted.

Thanks for your suggestions. We've water changed, checked the tap water, tried some other stuff and nothing's helping. I think right now, we have one fish who's on his last legs (the immortal pleco) and two fish who still check out OK. The plan is probably to see if the fish store will take the two healthy fish, wait til the pleco goes and start over. It's kind of sad-the pleco survived two aquarium changes and a week without power in the winter.

That may be the best idea. If you have gravel in the tank, you may want to save a little of it wet so you can restart the nitrogen cycle in the new setup.