Hagen

What is actually going on is what is called New Tank Syndrome, and actually is relatively common, especially in the spring.

New tanks are sterile, the water is conditioned with a water conditioner such as Aqua-Plus to remove the toxins that kill harmful parasites and bacteria in the water to make it potable - fit for human consumption. Once those toxins (often chlorine) are removed, the water is sterile, but able to support life. So life starts to make the aquarium its home...the fish you put in are only a part of the life of an aquarium.

One of the fastest life forms are heterotrophic bacteria, these are bacteria that have very fast replication periods and live in the water column. Since you started the aquarium in springtime, there is a good chance that there were lots of extra dissolved nutrients in the water as well as nitrate concentrations from the spring run-off as snows melted and rains washed the debris and wastes from the winter into the water supply.

Couple a fast breeding bacteria with lots of dissolved nutrients for them to thrive on, and you get cloudy water - what often appears as if a glass of milk was poured into the tank. The bacterial bloom is so great that you are actually seeing the bacterial population density with your naked eye. The bacteria are eating their way through the dissolved nutrients and exploding in population. In addition, they are using up a lot of the oxygen in the water as well, so the fish are forced to be near the top where there is the most oxygen.

Adding more food at every feeding also helps the bacteria to explode and since they have such short lives as well, they die off and decay as well, all adding to the overall bloom. Excessive nitrite, nitrate, decay and the water becoming matured can offer the smell you note. In addition, there is a natural increase in ammonia and nitrite that occurs through the life processes of the fish, breathing, urinating and defecating in the first month or so. These compounds are eliminated by another set of bacteria, labeled beneficial bacteria, but these need time to locate and populate the hard surfaces of the tank and the filter. They are part of the Nitrogen Cycle, and that is more clearly defined in our free download on the web of the Basic Aquarium guide.

Look for the Basic Care section and download the booklet for a lot more on filtration. It will give you an idea of what else is going on invisibly in your new aquarium.

What you are going through is natural, and it will pass, the bacteria will literally eat themselves to death as they use up the reservoir of dissolved nutrition in the water that you added to begin with. As the Nitrogen Cycle begins to establish, the smell will be eliminated as the bacteria remove the ultimate causes. You might want to hurry the process by not feeding the tank for three days or so, that will prevent any extra nutrients to be added and the bacteria will then die off to an invisible population. Not gone, but at a much more tolerable level.