I got this tank the day I got the gar. Tuesday of last week. The only thing I could think of is he got scared and hit the tank. So the qustion is do u think he will live.
I am not sure what I am reading here. If the tank was procured the same day as the gar that is now in it, how did you manage to "cycle" the tank that quickly? I am shocked that nobody here on MFK has asked this simple question.
Two days after purchasing and setting up the tank and doing a water test to determine if water quality is the culprit is ludicrous. A baby gar placed in the 20G as a growout is fine, however two days after filling the standard 20G with clean treated tap water testing is a total waste.
The fact that your water parameters in a 20G did not show out of acceptable range IN JUST TWO DAYS is BY DEFINITION.
Your water that you filled the tank with would need to be Freaking lethal to get "bad" reading in just two days!
Now back to the "nitrogen cycle" and you supposed water parameter readings:
You are one of two things here: Extremely confused and incorrect, or just not being 100% forthright!
Given that adequate B.B. seeded media was added to the 20G tank two days prior along with the baby Gar and a few feeders to accomplish the "quick cycle" and your water starts at 0PPM nitrates, than it is not possible for your tank to be cycled with BB and maintain a 0PPM of ammonia WITHOUT some rise to the Nitrates.
It is that simple. Beneficial Bacteria #1 eat Ammonia thereby lowering your ammonia reading to ZERO by increasing you Nitrite reading. Beneficial Bacteria #2 eats the Nitrites produced by the 1st beneficial bacteria driving your nitrites down and adversely increasing your nitrates in your water.
Nitrates are lowered by prolific plant growth roots in leaves out of water. This is not acceptable to a gar that WILL jump out of the tank given that kind of setup.
The other way is 1,000 times more common in the hobby, and is referred to as the water change.
Water parameter reading are valid only BEFORE you complete your water change.
You CANNOT have these readings short of changing the tank water immediately prior to the test. Even then I personally cannot get below 7PPM Nitrates reading straight from my well.
Do you have chlorinated water (public water supply)?
This is most likely caused by chlorine or as others said a broken back.