Tank Advice needed

[K24]

I bought a 24L tank in March and I cannot for the life of me figure out how to get my ammonia to sit at zero.

Tank info:
24L temperate (no heater)
Interpet 2200 Internal Aquarium Power Filter PF Mini
Filter has standard mechanical sponge (no carbon sponge) and I put pure bio balls in each week.
Currently no live plants.
No recent deaths.
9 white cloud minnows & 2 Amano Shrimp.
Have been putting Fluval cycle bio enhancer in (2ml) each water change.
Have currently been doing 25% water changes daily to try and keep ammonia down.
Nitrates are between 40-80.
Nitrates sit at zero at all times.
Ph 7.5.
Limescale is present in tank as we have very hard water in the area. I put interpet bioactive tap safe plus in with each water change.
I use the API master testing kit.

The fish aren't dying but I'm concerned that they are building up a tolerance to the ammonia and as a result may be becoming unwell.

When I set up the tank I did get given some mature filter media from a friends tropical tank but I'm concerned I killed off any good bacteria because I used interpet ammonia remover a few weeks ago because the ammonia was really high. That kept the ammonia at zero for maybe 48 hours before it came back.

Please help!

 

[Delapool]

Hi, one to look at is where are you cleaning tank or replacing anything in filter? This will be disturbing (eg extensive gravel vacs) or removing bacterial populations.

 

[K24]

Hi Thankyou for your reply.
I have removed all the gravel because I had tested it and it had been leaching ammonia into the water (cheap colourful gravel from large pet product store) but since removing it I still can't get the ammonia below 1ppm. When the gravel was I there is was reaching much higher.

 

[Delapool]

And nitrates are at 40 to 80ppm? Tank doesn't seem to be hugely stocked (although would cycle slower in cooler water) so I'm thinking the changing of the gravel (even though needed) would of removed good bacteria. Also what are the bio balls you put in each week? Is that into the filter?

 

[Delapool]

Also do you have ph reading?