I think using lemon juice is probably not a good idea. I'm generally of the opinion that trying to adjust your water pH is wasted effort at best, and possibly dangerous.
I'd try adding peat or driftwood to the tank and see if that helps. Also, a lot of tanks tend to get more acidic as they age.
Some people set up a bucket/bin/trashcan full of water and an airstone/pump, and add a bag of peat to that, and let it condition the water for a while before adding it to the tank.
If you have to add something, I'd go with as close to a pure acid as possible, you are adding all sorts of other chemicals and compounds with the lemon juice, and I'm not sure how they would affect other life in the tank.