This is probably due mostly to this being a brand new system that probably hasn't built up any trace nutrients yet. New leaves yellowing (while the veins stay green) is commonly a sign of Iron or perhaps some other trace deficiency. Now sometimes this isn't because the system is actually deficient in that element but more commonly the pH may be too high for the plants to make use of that element. The pH being too high does lock out Iron.
So, what is your system pH? How is your system doing on the rest of cycle up (nitrogen cycle, is your bio-filter working yet or is your ammonia/nitrite still spiking?)
A little bit of maxicrop or maxicrop plus Iron would probably help your plants out during this early period of the system. If your pH stays high you may also need to find a source of small amounts of chelated iron to add regularly to the system as chelated iron remains more usable to the plants even at the higher system pH.
If you have real difficulty finding chelated iron or maxicrop with iron, you might get some liquid iron and mix it in a spray bottle according to the directions and spray it on the plants as a foliar feed.
What I did to kick start the system was to use a bit of urin before I had even put any plants or fish in the system... Once I did that the ammonia and nitrites spiked and now there are a sufficient level of nitrates and nitrites in the system while ammonia levels are low.
I had and possibly still am having problems with my pH. It started very high (alkaline) and I used a bit of pH decreaser to bring it down. I probably used too much as the water became acidic after a few days. Now I've used a pH stabilizer to level it off again. At the moment it seems to be in the low 6's but the fish look more lively today.
Doing the research on the internet I've come across both iron and sulphur issues. I'm not sure which one it is that I've got the problem with.
Well what sort of acid did you use to bring down the pH?
I would recommend trying to keep the pH from dropping below 6.5 (at least with the API test kits you can't read below 6 so you want to make sure your pH is above that at least.)
Anyway, with such a new system it is probably best to avoid too much tinkering or additions. You want to get the ammonia and nitrite levels to stay down and have enough nitrate to keep the plants happy (this could be 0 as long as the plants don't show signs of nitrogen deficiency.)
At this point I think I would just recommend a little splash of maxicrop if you can find it, otherwise just keep an eye on the water test results and feed the fish for a while and see if the plants recover from the swings in pH.