Is my tank over stocked?

Right here's my set-up i could do with a little bit of advise as i am having loads of problems with nitrate levels rising and i am struggling to get them down i cleaned the gravel filter extensively last weekend and give the external filter rinse out but still the levels are at around 50mg/l + i also did a 50% water change and i currently do a 25% water change weekly .

Juwel 120ltr 1m x 45cm x 30cm
I have an under gravel fillter and rena xp3 1350ltr an hour
temp is 27c
pH 8 - 8.5
nitrite 0
ammonia 0

i have serveral plants(not rotting)

My fish

3 mollys
3 plattys
3 Dwarf Guramis
6 neons
5 cardinals
2 kribs
2 L199 caramel zebra plecos
2 x bristlnose young
2 x guppys
1 angel

please help

i would say your on the boarder line on your stocking.
be carefull not to clean the tank and fillters too good as you will just end up with more probs.
have you tested your tap water as the nitrate level might be high already so your just adding more nitrate.
you can get nitrate remover to go in your fillters. or do some smalll water changes with ro water if you can get your hands on some from your lfs and just re buffer it.
is your under gravel fillter one of those white plastic things or the grid type. when doing gravel vacs you can get all the crud from under the grid type but not the plastic ones which dont help.

its white plastic one

Yeah, tank seems to be borderline stocked. I would try to do more WC. I do 10% every other day on my amazon tank to keep the discus happy.

other think i spotted there is your pH...its a bit high. Most of the fish there like pH below 7. I myself never would consider keping bristlenose at over 7.3, 7.5 at the absolte max. Things like the neons and cardinals are amazonian fish and will die at that pH.

as Mooo said, be careful with cleaning. as you clean away all the good stuff too. it takes a while to find the sweet spot in terms of how much to clean. just keep at it

my tap water comes out at the pH. i car't find any decent info on getting the the pH down without using ro water i car't afford it . tried peat filtration and the pH drop to 6.5 over 2 days any iv been advised the bacteria stop working in acidic conditions so i took it out. got bog wood but the amount of water changes i have to do make the tannins negligible
also the person i got the bristlenose off live just down the road from me and was breeding them fine in that pH. they just all seem to be female