New to keeping fish - Do I have any issues?

Hello. I am new to fish keeping and would just like some reassurances and/or constructive criticism.

Whats been going on
I've attached a couple of pictures of my tank now I have set it up for fish - it used to house my axolotls so doesn't need cycling. It's a 90 litre (20 uk gallon) to which I have added some new stones at the back, some live plants and the large rock to the left (actually plastic). Water parameters are fine but the water in my area is quite hard. Filter is eheim aquaball 60 but I am going to upgrade to an aquaball 130 soon as I need more filtration to keep more fish.

The plants at the sides are fake and the plants in the middle are vallis. Water temparature is 25C (78F).

I first added 14 cardinal tetras, a couple of which were actually neons. They died one by one and developed white spot which I have now treated but only 4 remain, 2 cardinals and 2 neons. They have survived another week with no problems and are eating and I have just added 2 zebra nerite snails to help with maintenance.

I have noticed that the biggest neon is chasing the others quite aggressively, is it possible he bullied the weaker members to death? Or is he just letting the others know that he's the boss now there's so few?

Plan
Bump the cardinals back up to 10 or so
5 kuhli loaches
3 guppies
1 dwarf flame gourami
1 german blue ram
1 cockatoo apistogramma (or a 2nd GBR)
Maybe add another plant - Maybe anubias

Questions
Any ideas on why my tetras struggled early on?
Would my tank be overstocked?
Do you think I need more hiding places/plants?
Any tank mate issues?
I have no lid and a small light, is this a problem?

Thanks for any help.

James

Could you post all the numbers from your tests, please?

One issue I can spot right away is that most of your fish choices are soft water; if your water is hard, they're not going to do well.

Neons/cardinals are notorious for doing badly in new set ups, even if they are cycled. You should really add them only after your tank is properly 'mature' (which means it has developed a biofilm on the surfaces in the tank), which takes around six months.

Thanks for the heads up. The tank has been going for about 10 years with my axolotls in there with all the rocks towards the front, the log and the filter however I have added a few new bits to the tank recently.

I've been using test strips in recent weeks and cant remember the numbers off hand but am going to get a more detailed reading from my LFS tomorrow so will be able to update then.