Nitrates (NO3) have been studied and observations show some species of fish can withstand up to 100 ppm Nitrates without showing ailments. An aquarium should never exceed 30 ppm and a good safe level is around 15 - 20 ppm. If you have no plants in an established aged aquarium then 0 is just fine, but unlikely.
10 minutes
2x air + lighting= N02
Yes, most freshwater fish need a air stone in their tank. And even if they don't need one it will improve the fishes health.
-( 0 0 )- -| |- -(____)- <<<<< this is an example of your crab!
I filled the tank with freshwater.
Yes but you need to buy a saltwater filter to replace the freshwater one.
no because if its a freshwater tank the rain has salt in it
yes.
waterchange
It is carbon dioxide combined with ice and nitrogen.
Do you have the space for them? because freshwater angelfish are generally good parents. however, you should separate the fry by gender when they grow older.
Because he is from freshwater and needs to breathe freshwater instead if air.