I have a 55 gallon freshwater tank too. I have two Marineland Penguin Bio-Wheel Filters which puts it at 400 gph. Your Whisper 60 is about a little over 330 gph, which is not bad. First I must ask you, what type of fish. 12 danios or tetras or 12 platies or cchilids?
I have been at this hobby for almost 3 months now (I started on May 6, 2008). If you could, could you explain muggy? But let me jumpstart this reply for you.
First you must cycle your tank in order to get good bacteria that will feed on the waste, uneaten food, etc. In order to get this good bacteria, you must start off very slowly with fish. anywhere from 2 to 10 danios or tetras (not at full adult age or around 1' or so not at the 3" adult size. The key to cycling your tank is poop. Poop makes a good tank if done right. You don't want too much poop because then the bacteria that starts to grow can't keep up with all the poop which makes your water quality go down (this is what you are experiencing). I am unaware of your knowledge on cycling your tank. But again, to jumpstart this reply I will explain it as simple as I can.
There are 3 stages to a good cycled tank. Poop makes ammonia (highly toxic to fish), if you only have a few cyclers (zebra danios oare best because they are a hardy fish that can tolerate all types of water conditions). As the bacteria accumlates via the fish poop and uneaten food, another form of bacteria naturally begins to grow that eats or destroyrs the ammonia, this is nitrite. Nitrite is still toxic to fish (not as bad as ammonia but still toxic at moderate to high levels). As the fish poop more, the nitrite will be eaten by nitrate which, if kept at 0 to 20 ppm, it is fine, it can go to 40 ppm, but it is best at 20ppm or less), you will find the ammonia will go down to 0 and the nitrite will go down to 0, once this happens., that is, Ammonia 0, Nitrite 0, and Nitrate 20ppm or less, you have a cycled tank.
But you still can't add too much fish all at once because the good bacteria, that is in the gravel, filter media (only rinse gently your filter media, or else you will destroy any good bacteria), on ornaments, etc must remain, it must slowly build up to the point that it remains good bacteria, you never ever want to overwhelm the good bacteria (good bacteria or microscopic organisms that actually do live in your aquarium with the fish and live plants), so you must take it slow.
Understabnd? Fish = poop, poop = ammonia, good bacteria = zero ammonia & nitrite, and nitrate, at low levels (nitrate being the good bacteria) = a cycled or established tank. But you must, I respeat, you must do it slowly. I know you want to get more fish, of course we all do, it is a fish hobby. But we sometimes forget that fish are living creatures, and they can be killed by our actions, intentionally or not. So while you have a few fish that will cycle your tank, get to know the fish you do have, and in time, you can slowly add more fish.
I am unaware if you have live plants or plan to, but I would say yes to this. Live plnats absorb poop as fertilizer, it also absorbs nitrites, it also battles against algae (live plants will win out in a war with algae because eventually the live plants will exhaust the algae in a competittion for the carbon dioxide in the tank. More plants the healthy your tank, less algae.
The little thing I found after reading many books before getting my first fish was, to get about 10-15 ghost shrimp and 10-15 Olive Nerite Snails, these little guys will eat uneaten food and algae, which helps bring down the ammonia and nitrite levels. NOt that these little guys eliminate or quickly rids your tank of bad bacteria, but they sure do help.
I also bought 2 jars of aqua chargers which I placed in the filters, it helps to establish good bacteria in a quicker time period (I cycled my tank within 2-3 weeks).
Also one other thing, you must do water changes. A river flow constantly, the water is never the same in the same place in a river. This is the reason for water changes. I do 2 water changes a week, 9 gallons each time, on Monday & Thursday). In your tap water you may have high nutrients (by the way, you should have the following test kits: ph, alkaline, hardness, nitrite, nitrate, ammonia, oxygen at the very least...I have more test kits, but these are the essentials. I test the water twice a week; on Monday, I test before the water change, then I do the water change, then at night, right before I shut off the lights in the aquarium, I test again; then on Thursday, I test before the water change, but I do not test again unless I see a problem; oxygen I test once a week only if I see fish at the top of the water, if no fish at the surface, I do not test).
Good areation is a plus. I do not know the type of fish, and I hope you will supply this info, but if you have danios, tetras, etc, fast moving fish, good areation is a must, it helps the gas exchange at the surface of the tank (bubbles in the water is not oxygen, only when the bubbles surface and the molecules attach to the air above the tank, will the molecules turn into oxygen which the fish will tank in via their gills in the water...this is the reason I watch for fish at the surface because if the tank is low in oxygen, the fish go to the surface where the oxygen is plentiful, understand?)
So anyway, to make a long story short here, your muggy water is probably due to too many fish, not enough water changes, not enough good bacteria or it is just still going through the cycling phase. Some tanks take 2 weeks, some take 6 weeks. Patience my dear friend patience.
My advice is to get a good book like Freshwater Aquariums For Dummies (I bought this book before I even put one fish in the tank). It helps. Read read read. I can promise you, if you stay with it, do your water tests, do you water changes, do not overfeed, your fish will appreciate your company. I am starting to have my fish take bits of food from my fingertips, 2 weeks ago, they would not do this. Now they have begun to trust me. You know the funny is, fish are pretty good thinkers. They are not dumb. They do observe. They do interact, not only with themselves, but with us humans. Too bad I can't fit into my 55 gallon tank, boy would I have a load of fun :
So give it time, read a good book, and if you like, we can work through this or other issues via this format. I too started just a little while ago. I never take it for granted. I always observe, adjust accoridngly. I do not want any of my fish to die, to get sick. My platies are having babies/fry left and right, just this morning, I noticed 2 more fry. I have 2 fry that is one eighth inch, the other half of an inch; I do not use a breeder tank, I let the mother fish drop in the tank, and let mother nature take it from there. I feed the fry Live baby Brine Shrimp, they love it. As long as the adult or near adult fish are fed right, there is no reason for bullying the fry. So far it's worked.
So let me know the type of fish you have. We'll go from there. Also if you can, can you give me your most recent water test levels?
I hope the above info helped.