Lately I've been practicing with a slighly harsh voice. Much like yelling in a way, but not as forced. It's been getting better and all, but does it affect your clean voice on the long run? I'm not sure if reaching higher notes is going to be harder.

One good thing is that it has improved my endurance. But I wonder if that's all it does...

Well it shouldn't really affect your clean vocals.... Although if your are in the beginning stage with growling?! (harsh voice) you should be careful that you do the vocals right, otherwise some serious damages can occur on your vocal chords, which of course would affect your clean singing.

I'm doing low growls since about two years now... and I haven't yet experienced any negative changes in my clean singing, also reaching high notes is no problem for me....

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Just like Brutur said, if you do it right you should be fine.

I'm getting the impression that this "harsh" voice is something like Slayer? Or is it more like a growl? From my experience, I started growling without knowing anything about singing or how to growl right, which did some damage. But I was fortunant enough to learn everything fairly quickly and correct a lot of things, I got lucky. But now when I sing without growling, just natural metal singing, I sound like Slayer (which is NOT a bad thing ).
But if I truly do sing clean, perhaps for an accustic song (Opeth), my voice hasn't been affected at all.

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It all has to do with air....

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Yeah, it's not growling, more like Slayer. If it do it a few times I will be just fine, but if I keep doing it for a while(one or two songs, maybe more), it feels as though I may lose my voice on the next day, much like when you force large amounts of air through your throat.

Then I think you might be doing something wrong.

You shouldn't realy feel that way unless you've been screaming at the top of your lungs all night at some concert... we all know that feeling.

Do you have a PA system with good speakers? Cause I'd seriously suggest just turning up the volume a bit more and try singing from the stomach. If you do it right it should come out a bit louder than talking voice, when you actualy scream it'll come out much louder though. Put some reverb on, and possibly get a hold of a compressor.

Just some ideas. Also, a good point was made by powersofterror, learning how to breathe properly is deffinetly one of the things you should do now before you get realy far into singing. Ask around, maybe you can find a music proffesor, or anybody with bassic training, quire boy, chore, whatever.

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Ah, but it doesn't sound too loud if I don't use amplification. Plus I think I've figured out proper breathing, just the other day I was yelling something until I was almost out of oxygen(so I had to breathe real quickly), and when I stopped I felt a pain between my abs and lungs, much like when you lift excessive weight with any other muscle. I assume that was the diaphragm.

mabye all out screaming...not singing would help