Hey there Paul. Is the system 7 months old as well, or just the fish?
There are many reasons that plants will turn yellow. Most are usually essential element related. Some (particularly with basil) may be fungal related. Pictures and a description would be most helpful (i.e are only the newest leaves turning yellow between the green veins (Fe deficiency), are only the oldest leaves yellowing between the veins Mg deficiency), or both...or is the entire broad surface of the leaf generally pale and chlorotic...veins and all (N deficiency)? If the yellowing is sort of patchy on the basil, grab a magnifying glass and check the underside of the leaf, usually you'll then see a very fine whitish-grey fuzzy growth...
There are also a LOT of reasons plants will slowly die off...make sure they are not getting too wet and are getting enough light.
But if you think it's nutrient related, there are a number of easy and very inexpensive, fish safe fixes until things get (back) on track.
What are you feeding the fish?(Fish flakes make for some really crappy plant food). And is the system cycled, or are you in the process of cycling with the fish?
Oh, and what is the total volume of water that is in your system?
Ok, I'm assuming then that over the last year your bio-filter has been successfully oxidizing ammonia and that your plants were at least doing ok?...and that (from your previous low pH post) recently your pH was somewhere below 6 for some time and your bacterial colony has likely suffered some damage (bio-filters tend to crash below pH5.7 or so).
Take it easy on the feeding and test NH4/3, NO2, NO3 and pH daily. Watch for rising NH4 and NO2...
You might have to sort of "re"-cycle up the system. You really don't ever want your pH falling much below the high 5's...and since your test kit only reads to 6, keep it above that value or buy a test kit that can give you a reading below pH 6.