Just Starting The Money Pit Of Fish Keeping

Hi all, glad I've stumbled across this forum and Ill be doing plenty of reading!
Started with a little starter tank that I found in a school refurb that was getting thrown out and thought" Ill get it for the kids" I soon took over and claimed it as my own😂. I had 3 rosy barbs and a plec and a few plastic plants and a spongbob figure.
After scouring the web for a second hand tank I found a fluval flex 123l with a few rocks for £50 and snapped it up.
Ive since put in
2 new filters and media (didn't come with any)
New 150w heater
2 stones
3 plants (anubias nana, amazon sword and a crypt)
A sandy gravel Substrate (1-3mm)
A few patches of fluval plant soil just where the plants are planted.
I have 3 peices of drift wood soaking ready to go in.
Air stone
The old air fed filter and some old substrate in a stocking next to the new filters.
 
I got impatient with the cycle about 2 weeks in and my water readings were good and I caved in and got 4 harlequin rasbora To get the cycle going and some fluval cycle also put the pleco in there aswell after letting it acclimatise at top.

Its been 3 days and the ammonia has gone to 0.2 and no2 is still showing 0 but the no3 is showing about 20~25.
Plants are growing well also.
I no I should have read up first and should have waited the fish less cycle but any recommendations or tips would be welcome and thanks for having me here.
I'm looking forward to learning and growing my natural looking fish community.