I believe all plant varieties reduce nitrate levels a little but you'd want to check the Planted Tanks section for more info on that.
TBH a regular water change routine will solve your nitrate problems: I'd go with this method first off. I also agree with jchillin: do plenty of small water changes until levels go down. Don't just take out 50%+ water and replace it in one go b/c you will stress your fish with the sudden change in params.
On the pH: it's worth bearing in mind, btw, that on the pH scale the difference between 5.0 and 6.0 is a ten-fold increase which is an awful lot. Because it's decimalised I think sometimes people don't realise how large the scale actually is.
To raise your pH the water changes will help (bring it back to around neutral at 7.0) but if you needed it any different to that (more alkaline at 7.1+ or more acidic at 6.5-7.0 or whatever) then natural means are far better than using chemicals like pH Up or Down or Proper etc.
You can use certain rock to raise pH (e.g. ocean rock and the like), and driftwood will help lower it to slightly more acidic levels.
As someone said, a stable pH is adequate for fish even if it's not the 'correct' pH. Using natural means to buffer it in this way ensures any change is gradual.
Btw, your loach is probably going crazy b/c of the sudden changes in his params: every time I do a pwc
(even if it's just a small one) my schistura zip about all over and often flash as well. They're very sensitive fishies!