Koi fish?

Question
Hi,
I just inherited a fish tank (30 gallon?), filter(Aquatek 20-40), tank light, and two fish.  My landlord said his tenant moved and left it behind.  He asked if I wanted it and i said yes.  
However, he emptied out 2/3 of the water when I received the tank and the water was below filter line.  I am not sure what kind of fish I have, but they both seem very different.  I was left a food container that reads, "Tetra-O-Pond" Pond Sticks- Healthy Nutrition for Goldfish and Koi.

I don't want these fish to die.  They are both very beautiful.  I have spent the day adding water slowly to the tank.  The filter is now working, but the tank is still only half full and I don't want the fish to go into shock and die. I know very little about the care of these fish, but I have some general knowledge on the care of an aquarium.  Please help me to identify the fish, (I can send some pictures, if necessary) and how I can bring them back to a normal environmental setting.

The first fish is very large.  He/she is about 5" in length and has two sets of fins on the bottom.  He is one large fin on top and he is black and orange.  I don't know how to determine if he is a koi or a goldfish, and I can only assume that he is a koi based on the food left behind.
The second fish looks like a black angelfish. He is black and gray.  He is very small and has two vertical black stripes.He has two itty bitty fins in front, a very large back fin that is all black, and one top fin. His tail is all grey.  He is about two inches in length.
The two fish seem to get along, but I am not sure that they even belong in the same tank.

Please help me.  I will quickly respond to any questions you have, if you can get me going in the right direction with the proper care for both these fish.

Thank you kindly for all your help.
Sincerely,
Tricia Ransom

Answer
Tricia,
A picture would help me considerably but until then I can try. The first fish does sound like a Koi. If it is a Koi than the 30 gallon tank is way too small for it. Koi belong in ponds since they can get 2 feet long and sometimes even bigger.It is ok to fill the tank back up but make sure you are i=using a water conditioner before adding the water. The second fish I would need a picture of. I don't want to start guessing and it being wrong. If it is an angel fish, it should not be in that tank. Koi and goldfish are cold water fish, needing no heater, but an angel needs a heater. I will wait to see a picture of the smaller fish.