We recently bought a house with a 30 gallon hot water heater and at first everything was good with it. After a few months the hot water didn't seem to last as long as before, we also had the same size in our previous home with no trouble. We replaced both thermostats and elements and still a 10 minute shower was all you could get. We had a used 40 gallon from my parents home and put both new thermostats in it and new elements and now you might get a 15 minute shower if you are lucky! Anyone have any suggestions, I hate to go buy a new one if the water heater is not the problem, we are puzzled, please help!

Is the heat turned up to highest setting on the tank? They tell us that 120 is safe and effective. That may not be the case in your house. There are those who take "hot" showers that I find cold. Over 130? Can scald a person, but only if they're young (or immobile) enough to be unable to get away from the water and adjust it.

We had it turned up to 130 and it was really hot coming out of the sink so we turned it down to 120 and it is hot at first but gets colder after about 5 minutes and then is cold by 15 minutes.

Check dip tube. This is located on cold inlet side just under the nipple. Also, make sure that top and bottom thermostats are set to same temp. Make sure both thermostats are positioned snuggly against tank. If there is a gap, they will get incorrect temp readings, causing more problems than you would think.

I vote for mygirlsdad77's answer. Depending on how old the water heater is the dip tube could very well be your problem.
Have you noticed any little pieces of white flaky stuff in your water? Older model dip tubes eventually end up eroding and that is usually the first sign.

Because the dip tube runs almost all the way to the bottom of the water heater, that is where the cold water is supposed to go, so the hot water stays on top of the water heater... which is where it goes into the pipes of your house. If the dip tube is eroding, or possibly even completely breaking off, the cold water is entering the water heater more toward the top, which in turn doesn't allow you enough hot water as you should be getting.

Hope this helps.