Ammonia At 0.5ppm!

[Curiosity101]

Right well I did a quick headcount and no fish are missing.
No plants are rotting

Only recent additions were some frogbit (which is adjusting too my water as in i've lost a couple of leave but I've just removed them as an when)
And also a clutch of apple snail eggs hatched.
I had given the tank a good vac yesterday and removed any/all detritus
I did have a cuttlefish bone which I have just removed from the tank. It was in there to supplement the snails with calcium, and to hopefully up the KH a lil bit.

So any ideas? I decided it may be the cuttlefish bone so removed it just to be sure. It was a tiny bit black on the underside where it was buried in the sand so could this be it?

Obviously I'm dealing with the ammonia as I would normally with a water change + a bit of ammo lock.
Oh also it's been 4 weeks + since the filter was fully matured. I haven't forgotten to add de-chlorinator etc etc...so...

 

[WILDER]

Do you over feed.
Any rotting snails or plants in the tank.

 

[Curiosity101]

Nope, no overfeeding. They also only get fed 6 days a week.
No rotting snails I don't think. I had a very good root around in the sand. It's hard to tell with the newly hatched. But I'm pretty sure all are alive. (And they are only just over 1mm each)
No rotting plants. I remove any browning,thinning, unhealthy looking leaves as soon as I see them.

 

[WILDER]

Best to remove oraments every couple of weeks to do a vac undermeath them for waste and debris.
I would just keep doing water changes till ammonia reading back to 0.

 

[Curiosity101]

I do vac underneath them weekly. I know full well it isn't my tank maintenance routine that's caused this problem.
Which is why I'm asking could it be that the cuttle fish bone was decaying?

 

[WILDER]

I suppose it could be the bone if it was decaying.