Just as we put out bi products when we breath so do fish. They also poo and wee in their environment. These products of life become deadly poisonous Ammonia as they break down. In order for things like fish and plants to be able to live, Nature developed specific bacteria to change the deadly poisonous Ammonia into relatively harmless Nitrate in water. Then the nitrate can be used up as food for water plants and algae. A "cycled" filter is simply a filter that has been running for long enough to develop the natural bacteria that converts Ammonia into Nitrate. The over abundance of Nitrate can be removed by simply doing a weekly water change.
There are several simple to use 'Test Kits' available at most pet shops.
Ammonia levels should be at zero. Ammonia is never safe to have in an aquarium especially new fish. Transporting a fish generally causes some stress for them and with the addition of a water problem in their new home it could prove fatal. If this is a brand new tank it would be wise to cycle it fully before adding a fish.
All fish produce Ammonia.
Ammonia is harmful to fish. Its is a by product of the nitrogen cycle, which is what keeps the water stable in your tank(i wont delve into that) but yes ammonia can burn fish, especially those who lack scale's.
The recommended ammonia level for a dwarf puffer fish is 0 parts per million (ppm). Ammonia is toxic to fish, and even small amounts can be harmful. Regular water changes and proper tank maintenance are essential to keep ammonia levels at zero.
No. What happens in a fish tank is that ammonia is produced from the fish's waste (it is excreted through the gills, and their poo and uneaten food rapidly breaks down into ammonia). Plecos, by the way, are very messy fish. There is no way to prevent this from happening, and ammonia is extremely toxic to fish, even in tiny amounts. So the ammonia needs to be removed. The bacteria that change the ammonia into non toxic substances live in the filter, so without a filtration system the ammonia in the tank will build up and build up, killing the fish very fast. In order to sufficiently dilute the ammonia without a filter, you would have to do huge water changes several times a day, and in the sort of tank (100 gallons) required to support a fish as big as a plecostomus, this would be logistically impossible.
Yes, ammonia can not only affect the growth of fish, but if the ammonia level in a tank is too high, it will kill them.
There are three major reasons you can have high ammonia in your tank. First, you could have a dead fish in there. When a dead fish rots it makes ammonia. LOTS of ammonia. So, check your tank for dead fish. Second is overfeeding. The food the fish don't eat sinks to the bottom and rots, and rotting anything makes ammonia. Third is that you put too many fish in the tank too soon after you got it. There is a process going on in your tank called the nitrogen cycle. Not to put too fine a point on it, but fish pee has ammonia in it, like all pee does. There are bacteria in your tank that convert the ammonia to nitrites, then to nitrates, and finally to nitrogen which evaporates into the air. If you put in all the fish you want to have right after you get the tank set up, this cycle (which takes some time to get started) doesn't have a chance to get going and you get high ammonia levels. My advice would be to move the fish to a different aquarium--right now, a 5-gallon bucket with dechlorinated water in it and a power filter on it will work as long as you've got small fish--break down the tank, wash the gravel really well, set the tank back up, put "cycle aid" in it to get the nitrogen cycle started quickly, then put half the fish back in the tank in two days. After a few days put the rest of them back.
Ammonia
For a freshwater tank, its safe to add fish after the the ammonia, nitrite, and nitrate levels are safe for fish. Ammonia and nitrite should be at 0 ppm and nitrate levels should be very low, under 50 ppm. You should also make sure that other parameters are safe for the species of fish that will be living in the tank.
Leaving a dead goldfish in your tank will boost ammonia levels.. You dont want ammonia in your tank. It rapidly kills fish
No, leave the tank for several days, then get your water tested for ammonia and nitrite. Depending on the results, either get a FEW new fish, or let the aquarium run until the ammonia and nitrite are gone.