one of my plants has yellowish bottom leaves and a...

This can be nothing or a mineral deficit usually magnesium. To test usually flush for a day then add the mineral mix to the water and and other nutrients. I water with nutrients one day clean water for 2 days then back again. This way you won’t have a nutrient build up and it also gives plenty of minerals to the plant without over doing it.

It's just the first leaves dying off, no worries. You really SHOULD start a diary, though, so if you have questions/concerns in the future, we will have as much information as possible to base our advice on... if this leaf was in the middle of the plant, we'd have no way of telling what the problem might be! Good luck....