It seems that I begin many of these letters by quoting from some correspondence. After all, what better or more practical source of inspiration can there be? In this instance, the point is one that is brought up rather frequently by beginners in aquarium care, who have not been very successful after having seemingly followed the primary instructions gleaned from books or our dealer friends. They give recommended foods in conservative amounts, have good light and temperature control. But here is where trouble starts, through the acceptance of a fallacious signal as to what constitutes “overcrowding." The signal watched for is when the fishes gasp at the surface of the water, "blowing bubbles."That is a carry-over from the days when goldfish was King. Goldfish and other cool-water fishes are very sensitive to any shortage of oxygen in the water, or the presence of too much carbon dioxide. They quickly express their distress by breathing at the surface. Incidentally, I have often wondered how fishes, never before in such a situation, know enough to get a fresh supply of oxygen at the surface of the water.Warm-water fishes are better equipped to get along in oxygen-deficient conditions. In a tank containing both goldfish and exotics (a combination not recommended) the goldfish will invariably be the first to register discomfort from overcrowding. The point that I am stressing is that “Tropicals” are apt to "suffer in silence." When they come to the surface and stay there, conditions are not merely bad, but very bad. Undetected crowding has been present for some time past, indicated by the poor condition of the fishes. Of course such symptoms can come from other causes, but crowding is one of the first to look for. That suspicion can be confirmed if frequent partial changes of water relieves the condition.Water changes help keep the parameters within acceptable limits, help remove excess organic material such as waste and uneaten food, and also replenish required minerals in the water that the fish use up over time. If you prefer not to do as many water changes or are physically unable to, there are alternatives that can reduce your labor. My favorite is the EcoBio-Block, which is an aquarium care product that introduces beneficial bacteria into the aquarium (which keep the biological filter healthy) and slowly leach necessary minerals into the water to keep fish healthy and help beginners become successful aquarists.Advising a new aquarist at the height of his frenzy to go slowly in building up his tank of fishes is like talking against the tempest. Recently I fitted out a grandson with an aquarium and a suitable collection of fishes. All was lovely for a few weeks until he was bitten with the desire for more and more.The dealer could not be blamed for selling to him, but the result was not hard to foresee – a general attack of "Ich." Overcrowding does not necessarily cause that disease, but reduces the vitality of the fishes so that they are more subject to it.The elder Rothschild is credited with the wise crack "Nobody ever got poor taking a profit." I would paraphrase that in reverse: "No aquarist ever got into trouble by having too few fishes:"