if your fish have been fine for that long, they should be alright.
I have a 200L/3foot tank. I only had a very small sponge filter in it. Which probably did practically nothing in a tank the size of mine.
I didn't realize at the time how important it was to have a good filter, powerful enough to filter all the water.
I now have an "aquaone power filter"
I was the same. I spent about $100 on on fish. At between $3 and $6 each, that's a lot of fish over a period of time. They just kept dieing one after the other, like you said. Then my favourite one died. I'm in new zealand and I'd gone up north to Auckland (12 hour train ride). While up there I bought this tiny little orange and white fantail. I bought him back on the 12 hr ride in his little plastic bag. I thought he'd be dead when I woke up at my destination, but I looked in the bag and he was swimming around. I had him for about 4 years. He was always in little homes and I bought my new 3foot tank. Put him in there and he ended up dieing from the dirty water. I think it was probably a combo of disease and ammonia poisoning from all the poos. The moral is, I cried, he was call Mr. Fish
. He wad about 3" long when he died, and I can honestly say he was the brightest, prettiest fantail I've ever seen
.
So, I learnt a hard lesson
. Out of the 13 fish I always had and the 50 or so I went through, one survived. I still have it now. It's called Mrs. fish. It's a blackmoore/nymph
.
Sorry to ramble on
) . That's how I learnt that filters are important. But like I said, If your fish haven't died in a long time, they're probably fine. I'm really only a beginner and my powerfilter exp is new to me but I can safely say it does a really excellent job of keeping my tank water clean. I think that all filters work, just that you've gotta have the right size (big enough) for your tank.