Nothing stands out there. Except that if the plant is java fern it shouldn't be planted in the substrate. Maybe post a photo of it so it can be definitely identified.
My concern was that you may have had zeolite or some similar product in your filter as that would remove any ammonia leaving nothing to get the bacteria to grow. But as you have a U2, that doesn't have zeolite in it.
The fish in the tank all like low pH so that shouldn't harm them.
The fact that you have neons and rams mean your water must be pretty good as they are both delicate species which often just die, particularly if the water conditions don't suit them - pH, any ammonia or nitrite, high nitrate.
Prime and Stability shouldn't affect your pH - I needed to check that you weren't using pH down or some such chemical
However, you do have problems.
Using the community creator and substituting 6 panda cories for 3 julii, you are 117% stocked, and with insufficient numbers of the shoaling species. You need at least 6 of everything except the rams. But as you are over stocked already, you can't add more fish.
Neons are fine in a 60 litre tank.
Julii cories need a tank at least 90 cm long - though they are probably trilineatus (false julii) which would be OK in your tank. Most shops mislabel these fish.
Rams are OK size wise, but unless you have a bonded pair you could have problems with them fighting
Galaxy rasboras are fine in that size tank but you need a lot more of them. They do better in large shoals of more than 6.
Harlequins - fine in that size tank but you need more than 2
Scissortails - these are the real problem as they grow to 6 inches and need a tank at least 120 cm long and over 250 litres.
Details on these fish
hereThe best thing you can do is decide which of the suitable species you want to keep and get more of them, and rehome the rest. A shop may possibly take them off you if you get other fish from them.
If you do have a bonded pair of rams they will spawn and the cories will suffer. Cories have no sense of territory and will not learn to stay away from the rams' eggs. They will be picked on by the rams.
Once you have your stocking down to a sensible level with more appropriate numbers of the more suitable fish, you may find your water problems sort themselves out.