Tricky one this. The filter is there to harbour beneficial bacteria which break down the fish waste (ammonia) into less harmful....but still nasty stuff (nitrite) and then into not so bad stuff (nitrate).
Now filters can take a bit of time to get this bacteria....sometimes days, sometimes months, no set up is the same. Sometimes, the filter needs a bit of a kick start by introducing some "dirty" water from an established tank.
If you know anyone who has a goldfish or tropical tank, use a few litres from that tank and put into yours, it'll give the filter a good boost.
Keep the feeding low for a few weeks....just a small pinch a day and maybe add a small bunch of aquatic plants which might help to keep things moving nicely.
That's the theory behind all levels of fish keeping.....some fish are more tolerable of the ammonia than others but its not good to subject any fish to it for long periods.
Good luck