first of all i would suggest getting a water testing kit for at home instead of relying on the lfs. each morning before feeding the fish, i check the water. especially in the smaller tanks which are more of an upkeep than the larger ones. also if you ever have a problem with one of your tanks it is very helpful to know what the parameters are.
if you are having troubles stabilizing your ph, you might want to add some crushed coral to help stabilize it naturally. not really big on addin g chemicals to my tanks.
the three toxins that you want to watch out for are ammonia, nitrite and nitrates. ammonia is the more concentrated of the three and the most harmful. ammonia is created in the tank when excess food, fish waste and decaying debris is allowed to sit in the tank. ammonia is eventually b roken down to nitrites. nitrites are a mess concentrated form of ammonia but still can be fatal. the reason for cycling a tank is to create a biological filter in the tank which will help to rid the aquarium of these toxins. during cycling a nitrifying bacteria is created in the aquarium. this nitrifying bacteria feeds on the nitrites. the last is nitrates. nitrates are the lesser of the three. aquatic plants are great for the aquarium since they need nitrates, among other things to survive. plants feed on the nitrates. the faster growing the plant, the more nitrates are needed. slower growing plants do not require as many nitrates.
the only acceptable level of these toxins is zero. any trace amount of these toxins can result in fish loss. fish take in the water much like we take in air. if toxins are present the gills help to remove the toxins out of the body. a high concentration of toxins and the gills become "clogged" with toxins. the fish becomes less and less able to expel the toxins from their bodies which start to attack their systems. resulting in fish loss. gills also expel other things from the fish bodies.
letting these toxins rise also opens up your aquarium to disease and infection. that is why it is important to test your water. it is pretty simple to do on your own. the easiest way is to use the strips. take a sample of your water and dip the test strip into the water for a few seconds. take the strip and compare the color against the color chart on the bottle. if a level is detected it is time to do a water change. my larger tank can go about 10 days if i push it without doing a water change. the smaller tanks require more frequent water changes.
one misconception of the larger tanks is that they are higher maintenance than the smaller ones. this is untrue. the smaller tanks will have higher toxin levels more frequently than the larger ones. less water for the toxins to be dispersed into. i usually do water changes on my smaller tanks, 40 gallons and under twice a week or three times a week depending on the amount of stock. the larger tanks, 85 and up require less frequent changes. about once a week whether they need it or not. also larger tanks will help more with having a readily available tank. no need for cycling. if i want to set up a tank, i just do a larger water change on my 180 than usual. no need for cycling.
when you add fish you may notice the toxin level becoming slightly elevated. fish should be added slowly so the bacteria can increase to handle the larger load. otherwise you can overwhelm the bacteria and "crash" the biological filter of the tank.
if you have any other questions, just ask
semper fi