50 gallons, five years old. We were able to wash dishes, do laundry and take two baths. Recently it doesn’t provide as much hot water. A single bath exhausts it.
I’ve drained a few gallons off to see if it had filled up with mud, even though that is an electric heater problem. I can find no leaks. It still has a quick recovery.
Bottom heating element is burned out.
If it is gas there is no heating element.
Being gas it is pretty much an on or off proposition.
My guess would be the thermostat that senses the temperature of the water is on the fritz.
That said I am no expert on water heaters. Just a layman’s best guess. But since you still get some hot water out it would seem the gas heater is working but just not turning on enough to get the water hot enough. That would be the thermostat that regulates the heater (I would think).
Tl;Dr: You probably need a professional to come out and fix it. Expensive. (I just spent $250 on an HVAC guy to come out and fiddle with a few wires…sucks.)
What water there is is hot as hell. It’s like there were 20 gallons instead of 50.
Before calling out the professional, check to see if the temperature on the thermostat is set at the proper level. They don’t label them in degrees anymore, they just have something like “Normal” or “A, B, C, D.” Find the user manual online and see what the recommended level is and make sure it is there.
How did it get changed? Weird stuff happens.
If the water you do get is as hot as it ever was, then it seems to me that it’s not completely filling, or you may have a damaged dip tube. The hot water is pulled from the top of the heater, so when cold water is added at the top, the dip tube ensures that it is mixed at the bottom of the rank and the top remains hot.
With a broken dip tube, you will first get your normal hot water, but as the tank drains the cold water will mix with the hot at the top of the tank, cooling it down. You will then naturally turn down the cold water tap to get as much heat as you can, draining the hot tank faster.
Does that sound like what’s happening?
Yes, it does.
If the dip tube is the problem, the good news is that they are relatively easy to repair.
I found that, thanks!
Sounds like Sam is spot on for this one. There’s a good diagram at Troubleshoot and replace water heater dip tubes that shows what happens when the dip tube is broken.
Forgive my ignorance, but wouldn’t it make more sense to have the cold water inlet at the bottom of the tank? This has been the way in every tank I have ever looked at.
It is, it just goes through a pipe at the top to get there. 
Yes, it would. I imagine there is some traditional reason for that.
The guy who did plumbing for me took another job and quite free lancing, but bless him, he installed the water heater with a hose, so I can replace the dip tube myself, after I get to a hardware store on Thursday.