What happen to my all fishes, catfishes?

[Second boy due 3/4/09 <33 =]]

I am so upset all my fishes, catfishes died this week? First fish died I thought it's was old fish, next day two of another fishes died! I was confused, then next day catfishes, fishes died I was like what's going on? I have them for two to three years, But this week all fishes, catfish died? I don't understand? Of course I feed them right foods, I also feed them every skip a day. I have frog in there but this frog still alive, I am very confused and wonder why all the frogs, catfishes died? Now I only have one fish there left it's break my heart. I would said probably 10 of fishes died at this WEEK! Please help me, I don't know what wrong? & I'm very worry. Someone think it's could be infection there with germ from the first fish died that make every fishes, catfishes died from it?
Sorry I meant why fishes, catfishes died? not frog (frog still alive)
To Junie,

Sorry, here's your answer

What size is the tank? 20" wide,& 16" tall
Is it heated and filtered? Both yes & I always add new filter if the filter is dirty
How long has it been set up? I have them two to three years
How often do you change the water and how? I change the water when I see the tank is dirty, I pour the old water out and then put new water in there, I washed it before I add new water in there, I have been doing this since they all were fine and there always had 12 gallon of water in there.
What are the ammonia, nitrite, and nitrate levels? I am not sure, How do I find out those levels?

[junie]

We really need different details.
What size is the tank?
Is it heated and filtered?
How long has it been set up?
How often do you change the water and how?
What are the ammonia, nitrite, and nitrate levels?

Sorry, but without this information I really don't have a clue and neither should anyone else.

For water changes, get a gravel vac and only take out about 20% of the water. You should never change all the water.
Also, you can get test kits at your aquarium shop. They can get expensive, so if you are unable to afford one just take a sample of the water to the store. Most will test it free of charge. They should also be able to explain to you what the levels should be=

Good Luck

[TetraLinz]

You need a test kit to test for ammonia etc, but from the sounds of it, the fish have been dying because of chronic toxin exposure, plus poor tank maintenance.

Do you add dechloronator to the replacement tap water before you add it to the tank?

How much water do you change each time?

Clarity/cleanliness in a fish tank isn't everything. The important thing is correct maintenance. This means changing 10-25% of the tank water every week - regardless as to whether or not it looks like it needs it. Water in a fish tank can be spotless, but still needs cleaning every week-fortnight. After all, vodka is crystal clear - but it's lethal to fish (it's actually used in fish euthanasia).

Regarding the filter - there's no need to change it once it gets dirty - just rinse some of the media (stuff inside a filter) once a month in old tank water during a water change.