Hydroponic greenhouse - some problems

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I'm having a few minor problems in the greenhouse. I think most of them are caused by my watering pattern.
For the last 1-2 months my timer has been on 15 minutes on, 15 minutes off between 07:00 and 21:00 and 15 minutes on, 45 minutes off between 21:00 and 07:00. I have now changed it to 15 on, 30 off during the day and 15 on, 60 off during the night, hopefully this will sort out some of the problems.

I'm getting some cracked fruit on quite a few of the plants, this isn't a big issue really but still, I would like to get on top of it.
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And I'm also getting blossom end rot on some of the larger fruit, the weird thing is not all plants of the same variety develop this problem; the watering schedule is the same for all.
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Also on 2 of the buckets and on some of their fruit I find these black soil like things. They look like droppings of some sort, some are about 1mm and spherical and some are slightly larger and cylindrical.
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They're also present on a small number of fruit around the affected buckets.
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On the leaves of a pepper plant in one of the buckets as well. That plant had its leaves munched on by something also, I looked for pests on the underside of the leaves but I could not find any.
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Do any of you know what these black droppings are?

On a happier note my Zebra tomatoes are growing just fine and the fruit are looking awesome so far!
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My hungarian wax pepper has quite a few fruit set and one of them started changing colour and getting its characteristic sheen.
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Got some lovely tomato trusses, first one is the Sweet million and the second pic is Darkest tomato.
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Huge truss on this one.
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And to finish this off, I'm going to show off my largest squash this year, it's about a metre long now and still growing.
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Cheers for reading guys and any thoughts on my watering schedule or the weird droppings would be greatly appreciated!