klal or rebuy (Eruvin 27b-28a)

The gemara quotes the pasuk by maaser sheni that has a general statement followed by a limitation followed by another general statement. There is a four way machlokes tanaim what exactly should be included by that.
R' Yehuda Ben Gadish - you can buy anything besides salt and water.
R' Eliezer - you can buy anything besides salt and water and fish water
Braisa 1 - you also can't buy fish
Braisa 2 - you also can't buy birds

The last two shitos learn Klal prat u'klal (KPK) as opposed to the first two who learn ribuy miut ribuy (RMR). RMR is always much more inclusive than KPK. It assumes that you should include everything except the most basic thing that is completely dissimilar to the miut. There are two different ways to learn KPK that the gemara discusses. One is that the second klal is the main one so you start with PK which means that it's israbu lahu kol mili - it includes everything. Then the first klal comes and limits that. It limits it more than if you would learn RMR but not as much as if the other way of looking at KPK. The other way is that you look at the first klal as being the main one which means that you really have a KP which is ein b'klal ela ma sheb'prat and then the second klal can expand it but it's not going to expand it as much as the first way.

That's basically a summary of the gemara and it explains the machlokes between the last three shitos. Just to summarize:

RYBG and R' Eliezer - Ribuy miut ribuy - very, very expansive to include almost everything
Braisa 1 - klal prat uklal but start with prat uklal - somewhat expansive
Braisa 2 - klal prat uklal but start with klal u'prat - much more limiting

The only question that remains is what the source of the machlokes is between RYBG and R"E. It seems that RYBG should be correct. RMR should include everything except the bare minimum so why does R"E also exclude fish water? The fact that the gemara doesn't explain it makes me think that it's not a general machlokes but just a basic machlokes related specifically to this case. I think that the machlokes is either that R"E says that the exclusion is saltwater and if you have clear fish water that is no different than regular saltwater. RYBG maintains that if it's fish water then that's very different than just salt mixed with water. Another possible explanation is that R"E says that salt and water were never included in the original ribuy b/c they're not mazon. The ribuy only includes mazon so the miut has to exclude something that was initially included so he says that is the fish water. RYBG must hold that it doesn't have to be "mazon" to be included in the ribuy - just has to be food. (Before posting this, I just checked the Ritva and he seems to say like my second explanation which I like better also.)