Cloudy 55 gal. tank

Question
I have had the 55 gal. fish tank since November 06, we let the tank cycle for 3 days before we added fish. I know this was too soon, but I didn't know this at the time. We lost quite a few fish because if that. So we let the tank cycle and transplanted the 2 surviving fish to an established tank, a 10 gal. Once the tank stabilized, we put the 2 fish back and added some more fish. Right now we have: 2 plecs, 1 catfish, 2 angelfish, 1 very large goldfish, and 1 betta. They all seem to be getting along with each other, except when I put the betta in the goldfish tried to eat him, and the angelfish picked at his fins. the angelfish do that anyway, right? But now they pretty much leave each other alone.
About 2 weeks ago the water started to get cloudy, and I have an ammonia spike in the tank. I have AmmoLock to help control the ammonia somewhat, but it does not seem to be helping. We have done water changes, 25% each time. I bought a new filter, an expensive one, 2 days ago. the water was definitely alot more clear yesterday, but today it seems worse than ever. The new filter is a cylinder filter. I'm guessing this could be because there is no biological filter established in the new filter, so we threw the old filter back on there to see if that would help. I also bought StressZyme and Stress Coat to help the fish get through this ok. I have had many tanks in my life, but this is the largest and is giving me some trouble. None of the research info I have found has helped at all. Please tell me you have some helpful info for me.
~Katelin
P.S. the tank temp is approx 78 degrees all the time, and all the fish seem to like it at that temp.  

Answer
HiYa!

First off let me start by saying I'm not a fan of putting Betta's in community tanks. Yes Angels are nasty fish (I am not a fan of them) but Betta's tend to get picked on and bullied in a community environment and it is not best suited for their requirements. They prefer small tanks, with little or no water movement.

The problem you are experiencing with your ammonia spiking could be to do with your stocking level in your tank.  One Pleco requires a bare minimum of a 55 gallon tank to itself to reach adulthood and you have two in yours plus other fish. With them being large waste producers as well, you either need to possibly looking at getting a bigger tank, moving some fish or committing yourself to doing 25%-35% water changes every 4-5 days.

Hope this helps!

Cheers! Rach