Hi guys,
I have one of those aquarium corner filters that you put inside the tank and hook up an air pump to and it pushes water out the bottom and new water flows down through the top through the filter floss and charcoal. I believe it is a Lee's one without an airstone. I have the air pump set to high (meaning all the air is going through the filter and not anywhere else). The pump was made for a 20-40 gallon tank I believe but I have it in a 15 gallon tank. I have one comet in there (I realize this isnt the best scenario but it sure is better than what most people do). I regular water changes. I use a cup that holds about .5 liters x's 3 refills (I used a poland springs water bottle to check and it fills it up 3 times, so I'm assuming the cup is a liter and a half?). I do this every 4 days with water that has been sitting out in a clean bucket for 24 hours or a little more usually.

My question is how efficient is a corner filter? Especially one where all the air is being forced through it? Is this good enough for a goldfish? I used to use this in combination with a whisper power filter (that hung over the edge, but the filter is starting to die).

If anyone can offer some comments I'd appreciate it.

It would be hard to estimate what the gph would be on a set up like that.

However, I can say that when I first got into fish many years ago, that was really the only type of filter you could buy. I kept many fish, including goldfish, healthy using those types of filters.

What are you using as your filter media?

I think the most important thing here is water changes. You are doing them every four days which is great. You will have to keep up with that.

I wasn't too clear on whether you were changing a total of 1.5 liters per changes or 4.5 liters per change.

I would go up to around 30% of the water in the tank with every change.

Also, if you remove the Whisper filter, you will have to keep a close eye on things, since you will be removing a lot of your filtering capacity, and it will take a while for the remaining filter to pick up the slack. You may see a ammonia or nitrite spike at that time.