Unexpected lotus plant leaf color

Question

I sprouted 4 lotus plants (Nelumbo nucifera) in the start of May this year.
I expected the leaves to be green like the one you see at the bottom center of the image below. However, as you can see from the attached images, the leaves display a wide range of colors. One is even pink! Please note that this started with the very first leaf, when the seeds were still in a tall glass vase.
Is this normal, or is there something wrong my my plants?
If the latter, what can I do to cure the problem?
Additional information:

Growing medium: Aquarium sand.
Container size: 70 L (18.5 gal)
Fertilizer: None
Sunlight: 7h direct + 9h indirect (59°N)
Air temperature : 15-30°C (59-86°F)
Water temperature : 15-40°C (59-104°F)
Water quality: Voss water (aka. Norwegian tap water)
Store: Seeds purchased from Plant World Devon Ltd

Update #1
~5 weeks after applying slow release fertilizer:

Update #2
~7 weeks after applying slow release fertilizer:

blacksmith37 · Answer

Water lillys and I expect lotus are heavy feeders. What is wrong is no fertilizer and poor growing medium. Because you have no koi or goldfish to dig, put 2"/ 5 cm rich garden soil in the bottom, cover with sand/gravel. And for good measure, mix a teaspoon of fertilizer into the soil first (I like 9-45-15 , but not high N lawn fertilizer).
The temperature seems high but I doubt it would hurt the plants. I have 3 water lilies blooming in my 5 X 10 ft. pond now with water at 84 F, mine will get to 90 F. If I was going to change something, I would use a deeper container for more thermal mass (lower maximum temperature). And more room for substrate, then you could add coarse gravel so gold fish can't dig, and add gold fish (they will probably eat algae). I have 2 koi about 5 pounds each and they do not mind 90 F (I do have aeration).

Unexpected lotus plant leaf color

Update #1

Update #2

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