I just recently was given a 40 gallon Hex tank. I am very new to the aquarium game and am looking for some guidance. My first question is what type of filter to purchase. The dimensions of the tank are 22x24x25. It came with an under gravel filter plate and two tubes, however no pump or anything. My second question is how many fish and what kind of fish would be ideal to keep in the tank to ensure healthy, happy lives for them as well as a community aquarium feel. My third question is what type of substrate to use, sand or gravel? And finally my fourth question is what type of heater to get to keep the fish happy. Thank you very much in advance for your help!!