A pH of 8 is alkaline and high but fine for mollies (but a bit high for neons). The only exception to this is if there is ammonia in the water. In alkaline water (pH above 7.0) ammonia becomes very toxic to fish and other aquatic organisms, (shrimp crabs etc). However, since the tank is cycled and there was no ammonia it is more likely another water quality issue or stress.
Your tap water probably has a high pH too. Do a pH test and ammonia, nitrite and nitrate test on the tap water that you use to fill up the tank with.
Check the nitrates in the tank as well because high levels of nitrates can stress fish too.
If your tap water has a high pH then just accept it and leave it as is. Do not bother trying to adjust it. The neons will have adapted to the higher pH and the guppies and molly are fine in alkaline water.
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Do you have a picture/ aquarium backing on the tank? if not perhaps put something on the back to make the fish feel a little more secure. You can use aquarium backings from a petshop, or coloured cardboard, even plastic bin liners. Just make sure they are free of chemicals and tape them to the back of the tank. Dark colours show up the fish better unless the fish are dark.
If you don't have many plants in the tank, perhaps add some floating plants like Water Sprite (Ceratopteris thalictroides). It can also be planted in the substrate so is versatile
Make sure nobody is spooking the fish. Children or cats sometimes tap the glass and can stress fish.
If the fish tank is in a quiet room that is not used regularly, then get a small 20-30cm oscillating fan and tie some coloured streamers (about 10-12inches long) to the front of the fan cage. Have the fan in the room and turn it on randomly throughout the day. Let it run for 30minutes or so. The fan will blow the streamers around and this can help the fish get use to things moving around the tank. Having the fan oscillate (go from left to right and back again), varies the way the streamers move and makes it more effective.
You can also have a small television on in the room, you don't need the noise. The flashing light and movement on the screen can help the fish get use to things moving around them.
In the morning turn the room light on or open the curtain. Then wait 30-60minutes (or longer) before turning the fish tank light on.
At night turn the room light on, then turn the tank light off. Wait 30minutes before turning the room light off.
Fish do not like being in a dark tank and then instantly having a bright light turned on above them. It's a bit like someone shining a spotlight in your eyes when you are asleep. And at night they will sometimes panic and dash about madly if the tank goes from bright light to complete darkness.
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How often do you do water changes and how much do you change?
Do you gravel clean the tank?
Try doing 75% water changes each week and a complete gravel clean each time you do the water change. Sediment in the gravel can encourage health issues and big water changes can help dilute water quality or poisons.
Make sure any new water going into the tank is free of chlorine/ chloramine before it goes into the aquarium.
Make sure you don't have any moisturising creams, perfume, etc on your skin and use a perfume free soap. Rinse hands and arms with fresh water before working on/ in the tanks.
No perfumes, deodorants, nail polish, paint or smoke near the tank. Anything that creates fumes can stress or kill fish.
Make sure you use a "FISH ONLY" bucket and ensure nobody uses the fish buckets for any purpose besides the fish.
