So lemon juice is citric acid. A weak organic acid. This is from Science in Hydroponics:
Common food grade organic acids (citric acid, acetic acid, etc): Organic acids are a very low cost way to lower the pH of a hydroponic solution as many of these are available off the shelf in super markets in food grade qualities. The main issue with organic acids – which anyone who has used them has probably experimented – is that the effect of the acids does not seem to hold (pH goes up quickly after the acid is added and the solution comes into contact with plants). This is actually caused by the fact that plants and microbes can actually use the conjugated bases of these ions nutritionally, causing an increase in pH when they do so. The initial addition of say, citric acid, will drop the pH – generating citrate ions in the process – these will then be absorbed by microbes and plants, increasing the pH again rapidly.
The use of these acids is therefore not recommended in hydroponics.
Cool website if you want to look under the covers of how PH works
Bottom line, if long term stability is what you are looking for, stick with the PH Up and Down products for hydro or fish.
If you are really feeling froggy, start with a potassium silicate product like Agsil16 first to send PH up, then counteract with PH down to set at 5.8. The end result is MSA in plant ready form (yum - awesome molecule for plants) and a buffered PH capacity that actively resists changes in PH.
Forget all this if you just want an easy 2 week cloner run, just use PH UP or Down as needed. Profit.
