Yellowing leaves on seedlings

Thanks, I'll get onto that- will post more pictures if the situation changes

You should put a small fan blowing some wind on them if you haven't already. That will knock them around and strengthen the stems; otherwise they will fall over when bigger.

Thanks for all your suggestions- after a while when it looked like they wouldn't pull through, they are now thriving!

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A few drops of vinegar in your water will bring your PH down, for two plants buying distilled water isn't a big deal but you could also boil/microwave the water as well then adjust your PH levels.