Yes. 2 Dempseys can survive in a 40 gallon tank. Will they be happy? No. Will they be stressed out? Probably. Will their growth be stunted? Most likely yes. BARE MINIMUM of 55 gallons for 2 JDs. Bigger is much better.
Yes. A pleco can live with an oscar. You just need to keep an eye on the...
You don't want any nitrite in your tank. It's not a good thing to have. If the fish you have in the tank are still alive in another week i'm sure the tank is cycled.
do you have a bubbler/air pump in there? he could be gasping for air. if they dont get enough oxygen they try to breathe faster to compensate. my jack does that when hes stressed out. he gets really panicky sometimes but this will stop. just leave the tank alone and he'll calm down.
it should be kept at a stable 82 degrees. you dont really have to clean anything other than the glass which you can do with a simple dish sponge...rinse it well to make sure there are no chemicals in it. feed salt water fish food flakes and brine shrimp. you can buy the shrimp frozen or freeze...