Fish dying in community tank

Question
I have a 5 gallon tank with a charcoal filer and a bio wheel. It has been cycled a few times now, I've had it since last September. I change the water (25%) every other week and feed once a day.

In the tank are are two guppies, 1 male, 1 pregnant female; 2 apple / mystery snails, 1 algae eater (I think he's a Chinese algae eater), and two dwarf African frogs.

Earlier during the Superbowl, I saw that one of my frogs was cartwheeling through the water under the filter stream. He was dead. He had been fully alive and active this morning. I took him out and studied the other fish. They all seemed fine (swimming normally, breathing normally...). Not even an hour later the female guppy was laying on the bottom of the tank dead. As I was taking her out, I saw an oily coating on the top of the water. I did a 50% water change and the oily substance was gone. However, during the change, I was looking at the walls of the tank (I noticed a slight algae growth) and saw little white flea shaped mites crawling in and out of the waterline. They are perhaps the size of a pin tip. Very small. I did not see very many of them, nor did I see them swimming in the water or on the gravel bottom. They were only near the top of the tank and the water line.

I can't find anything on them online. Could they be a parasite? The fish aren't injured with lesions. Water fleas? I'm not sure.  

How can they be treated, and could they be the cause of the deaths of the dwarf African frog and the guppy, or could it have been the oily substance on the top of the water?

My only guess to the oily substance is that we plugged in an air freshener to an outlet across the room from the tank. Could that have been the cause of the oil? I had not noticed the oil before this morning.

Any advice as to the mites and cleaning up my tank? I'd greatly appreciate it.   Thanks!

Answer
Hello Stacey,
It seems possible that those little white parasites you are seeing are nematodes. The only way to be rid of them is to deplete their food supply.
Fast your fish for about 48 hours, and cut back on how much you are feeding them.
You'll also need to start doing water changes more often. I would suggest 30% once a week. Otherwise the ammonia/nitrites/nitrates in your tank are going to build up and kill your fish.
You can get your water tested for free by some pet stores, and they can help you out as well. You should consider also purchasing a small gravel vacuum and cleaning the crud out of your gravel, because leftover food, and fish poop accumulate in your gravel and will add to the ammonia and is also a food source for nematodes.