I am getting all set to build our first AP home system. In the meantime, I thought it would be a good idea to buy a traditional table top fish tank, to get used to taking care of fish. Where I live (Colombia, South America) there aren´t as many supplies available for these hobbies as in the US., so I could not get pH or ammonia testing kits. I simply bought the 5.5 Gal tank, which came with everything needed: pump, filter, light hood; some gravel and deco plants and objects, and of course, four goldfish of various sizes.
Things went well at first. Then I realized that pumped water was bypassing the filter pad. Noticed the filter pad was saturated and rinsed it a couple of times. A couple of days later, all of a sudden the water went really dirty. And then the crisis came: the three bigger fish were gasping for air in the water surface, but the smallest one was already floating sideways and barely moving his mouth.
I had to go for emergency measures, or else find a good story for my 5-year old, formal owner of the fish, and who soon would get back from school!
My first guess was that Dissolved Oxygen was low. Kit did not come with an aerator pump. Store clerk told me it was not needed given the size of the tank. With a large cup I started taking water from the tank and pouring it back in, from as high as I could. I could see a lot of bubbles forming in the water. Repeated this several times, for the next 15 mins. Interestingly enough, in one of these repetitions I did not notice that the little fish (the one floating sideways and barely breathing!) went into the cup. When poured the water back, I saw him rush into the fish tank...which seemed to have awakened him. If I didn't feel so sorry for this guy (or ashamed of sharing this with you!), I would dare claim this is the closest you can get to CPRing a fish!
I then replaced almost all of the water. Did so in 1/3 intervals and repeated the exercise three times. I know this does not account for 100% of the old water since after the first time I had already fresh water mixed with dirty water, but it certainly seemed to have changed things for my little friends. In every amount of fresh water I added water conditioner, knowing that I should have let the water rest for at least an hour, but I opted not to do it. My fresh water does not have a lot of chlorine since it comes from a well.
Finally I pressure-washed the filter pad and placed it back. It seems to be working OK.
Intuitively, (remember I still don't have the testing kits), my guess is that the following happened:
At the end, after a lot of manipulation, repeated waterfalls into the tank, and a likely BRUTAL pH swing due to the abrupt replacement of water...our fish seem to look happy and back to their normal routine...even the smallest one looks again like a happy (but shaken) fellow!
For the time being, I am cutting back on the feeding until I manage to get hold of an air pump, a pH testing kit, and figure out what to do with the filter. Of course, one likely solution is to place a media growbed over the fish tank and create a small (maybe too small) AP system. This would certainly take care of water filtration.
If there is any doubt fish are resistant dudes, this note shall serve as testimonial they are!
There are some really tiny AP systems running quite successfully, so even though your tank is small you should get your growbed asap. It will look after the fish waste better than any purchased "biofilter". The growbed media provides a surface for the bacteria to convert ammonia and nitrites and the plants use up the nitrates and give you free food to boot. It also oxygenates the water twice. When the water returns to the tank the falling water adds air from the surface (just like you did with a cup, and how it works in nature in a river). But more importantly, a flood and drain system collects air from the surface of the grow media during the flood stage and takes it into the tank on the drain stage while the media again becomes coated in air for the next round. (A lot like the tide coming in and collecting air from the surface of the sand and rocks that were exposed during the low tide).