New-to-me aquarium

Question
Hello, thanks for taking the time to look over my questions.  First, I'm not experienced in fish tending, so these may not even be the right questions to be asking. . . Here's the situation: Yesterday I was given a fish tank that for complicated reasons had been abandoned.  It only has about 3" of water in it(the same water that the fish were living in presumably with no problems)and the pump/heater, rocks and fake plants aren't in there.  But the fish still are, along with their gravel.
Can I add more water safely?  I don't want to shock them, but they obviously need more than 3".  
Should I clean the plants and rocks before I put them back in?
What should I get for them to make them happy?

Details of the aquarium: 36"x18"x16"~44gal?  I really don't know if it is freshwater or salt water, but there's a blue crawdad looking thing in there, which makes me think freshwater??? Who's inside: about 12 small, thin, reflective fish that are silver with orange toward their rear and a horizontal neon stripe.  3 white fish, about 1.5 " long, sort of bubbely shaped, one has the whiskery things around his face and stays toward the bottom, and has slightly pink tinges.  3 fish that are about 1" long, classic fish shaped, with orange fins(top and back) and a black triangle on their sides, with one point reaching down toward the back fin. A black fish with an orange tip on his rear fin, seems to prefer hiding in dark areas, about 2" long.  And the blue crawfish thing, maybe 3" in length.
Thanks so much for any advice you have- I really don't want to accidentally hurt these little guys!

Answer
Hi Sara:  the tank is a 45 gallon tank.  It sounds like you have a freshwater tank and I would definitely add more water to it.  The small neon fish are probably glo-lite tetra.  Not sure about the white fish... maybe a catfish of some kind. The fish with the triangle are probably t-bone rasporas or maybe lamb chop rasboras.... I would definitely add water to the tank... You should visit the local aquarium store and get tropical flake food and water conditioner.  Then add in the water to the tank... It wont hurt them too much more to just add the water to the tank so long as the water you add is about the same temperature as the water that is in the tank.  For right now I would not clean the rocks and plants... I would do that in a day or so... if you invest in a gravel vacuum... which is about $9 at the local fish store it will save you a lot of time and energy.  They will need a filter too...  This is going to be expensive so let me give you some advice and a shopping list.

The tank will need:

A hood/light
a filter
a heater
and probably a bubbler ( air pump)...

These do not have to be purchased all at one time... so the first thing your tank is going to need is a filter.... I like the whisper filters that hang on the back of the tank.  It is important to make sure you have one that is rated to 45 gallons.  The second thing your tank will need is a heater... I am not sure where you are but here in California it is hot in the day time and cold at night and those conditions lead to fish sicknesses.... such as ICH.   If its warm day and night then get a light and hood and not the heater.

The bubbler can come last as the filter will add oxygen to the water... so when you put the filter onto the tank make sure the water from the filter falls in to the tank at least an inch before it hits the surface of the tanks water.

The fish are going to need to be fed and you are going to need to pick up some tropical flake food and some sinking carnivore food... the carnivore food is for the crayfish... otherwise he will eat your other fish... I suspect this is why there was only three inches of water in the tank.  Some of the fish  you have described are kind of pricey where I work... so I am a bit puzzled as to why someone would treat them like this.  The crayfish would sell for about $20 American and the rasboras for about 6 fish for $22 American... I would in the meantime do some Internet research on the fish names I have given you and if you can send me a picture of the white fish I will try to identify them.  You can e-mail them to me at davids@the-mermans-garden.com  ... If you have other questions please let me know.... dave