Correct me if I am wrong but don't Xenopus live in pretty hard alkaline water?
I would not be messing with the pH by adding highly acidic lemon juice unless the tanlk is really big (say one lemon per 100 gallons).

And for that matter, though they are adapatable, don't most gouramis come from neutral to slightly acidic water?
Not the best combination in my opinion. But i guess gourmais are gentle.