Hello pineapple,
I don't know what pictures were taken when because all three links are the same pictures. But I've got a few concerns.
First in one of the pictures there seems to be something on the stem of the one growing in the orange cube. I can't tell from the picture for sure, but it looks like it could be stem rot. It could just be
nutrient salt buildup, but the part of the stem where the problem is, looks thinner that the stem above that point. Stem rot would be caused by constant moisture on the stem.
Second, It looks like an air line in one of the pictures, but I don't see any air bubbles in the water.
Third, You said temp was 28-33 Celsius (82-91 Fahrenheit). I don't know if this was supposed to be air or water temp's, but the water temp will be close to the ambient air temp, and even 82 Fahrenheit is getting high for water temp (optimum water temp is between 68 and 72 Fahrenheit). I don't know what type of plants those are, but if they are a type of lettuce 82-91 Fahrenheit is a bit warm. Lettuce is a cool weather plant and goes into survival mode and bolts when temps reach near the 90's Fahrenheit.
Fourth, You say your pH is around 5-6. While plants are adaptable you might be letting it get a bit to low. Try for a range between 5.5-6.5. Try not let it go below or above that range. If you have to adjust it frequently, something is wrong.
Fifth, You said your using an A B solution for nutrient. But that doesn't tell me anything. That doesn't tell me what brand your using, if they are even hydroponic
nutrients, how they should be mixed, how your mixing them. I cant even look up a mixing chart for them etc...
With that said, 1500 ppm may be to high, but I don't know what water source your using, and as I said I don't know anything about the nutrients your using. Typically small plants like in the pictures should only get 25% to 50% strength nutrients.
With those concerns said, I have some other questions:
1. What are you using for your water source?
2. What nutrients are you using, and are you mixing them according to manufactures instructions?
3. What is the water temperature (both night and day)
4. How often do you check your pH, and are you using pH drops to test it?
5. Are you using a air
pump and air stone/s in the reservoir? How much air are you getting to the roots?
6. What is the humidity level?
7. Are you growing in artificial or natural light?
8. Why is the floating raft covered in black plastic?