urgent help needed I AM NEW TO THIS

Question
my dad had a red fined/rainbow shark and he was atacking his other fish. I have put him i an aproxamatly mesuerd 4 gallon tank (L)15.5" (W)7.5" (H)8" with a water filter running at about 23c-24c(NO AIR PUMP YET  i have one but is it nessasry with one fish) WILL HE BE OK?, as i want to keep him he seems to be quite shy as most sites i have visted have stated. he dose seem to spend some of his time virtcal against the water pump about mid level up the tank why would he do this? the rest of the time he spends resting at the botom of the tank or hiding behind his rocks darting in and out now and again

the main thing is that he appers happy and has not lost any colour or condishion and his sacles look fine he has been in the setup for a few days now i just want to check that this is ok

Answer
Richard,                                                     Number one your problem is your tank is much to small. Is it heated.... He should be heated to about 76 degrees . ...Who sold you a 5 gal tank for tropical. you should never go smaller than a ten...Good Luck Tina