Hello, I am new to the world of fish, escept for an occasional trip to Red Lobster,

. I have a tank which is built in between two rooms, it was custom ordered to fit the wall and is 175 gallons. I inheritted the tank when I divorced two years ago. I have three fish, 4" angel - ghost knife - alge eater. All I have done is feed the fish, I did add water once and clean the filter once. The tank is about three inches low of water at this time and I know it needs attention, what is the best action to get back on a routine schedule so I do not do too much to fast to hurt the fish. Actually the fish are doing fine at this time and the tank is not looking too bad taking in account the lack of attention it has received. I live in a small town, the pet/fish store employees have not been any help and only suggest that I buy all the chemicals they sell. Thanks for the help.