and try not to use chemicals as they will add to the osmotic pressure in the tank and cause more stress on the fish
Ok, I'm sorry if this is off-topic to the OP, but I feel that this statement needs to be corrected.
Firstly, what is right, the addition of chemicals or anything dissolved in the water, does indeed increase the osmotic pressure of the water. That is a fact that can be found in any physical chemistry textbook.
However, the internal fluids of a fish also have dissolved minerals in them, and in fact, usually at quite a higher concentration than their surrounding water. So, actually the addition of chemicals to the water, brings the tank's osmotic pressure closer to the osmotic pressure of the fish. It actually lowers the stress on a fish -- but I am specifically using stress in the mechanical sense of the term, like the pressure stress or the shearing stress.
Now, this is actually not really a good thing for the fish. It lived it's entire life with this osmotic stress/pressure, and so did all of it's ancestors. The fish evolved in this ostmoticially stressed environment. It does not need to be relieved of of this stress, any more than humans need to be relieved of the stress of the atmosphere pushing on us. People actually get elevation sickness when they travel to high altitudes in no small part because of the lack of pressure. (Yes, the lack of oxygen too. But, they did the pressure experiments with the astronauts to see if they would have to completely pressurize the space vessels. For them not to suffer altitude sickness, the answer is yes, they have to pressurize the space vessels pretty close to 1 atmosphere of pressure). The atmosphere pushes down on us with 14.7 pounds of force per square inch. I used to write find a 15 pound 1 inch cube and put it on your leg and feel just how heavy that really is, but I thought of a new one just the other day. I was holding my fiance's cat, who weighs about 16 pounds. If you hold him so that only one paw -- which is about one inch square in area -- touches you, you can feel just how heavy one atmosphere of pressure really is. But, we've lived with that pressure, that stress, out entire lives. We've grown adapted to it. We don't need to be relieved of it any more than the fish need to be relieved of the osmotic stress.
You don't need to invoke osmotic stresses to recommend against putting chemicals in a tank. The simpler reason is just that adding chemicals is unnatural, and most of us keep fishtanks to try to keep a little slice of nature in our homes. The chemicals can sometimes wear out (like a pH adjuster) and cause the tank readings to fluctuate. Other chemicals can do their job well, but stimulate large growths of algae.
It is usually more hassle than it is worth, because fish are very adaptable. Especially the common fish you have, fishfriendlyones. They are adaptable to a very wide range of water conditions -- you goal is to keep those conditions constant. Fluctuating conditions are stressful on fish (and here I am using stressful in the sense of weakening their immune system, NOT the mechanical sense).
