You have received tons of good information in this thread but
The high ammonia concentration is due to the lack of adequate biological filtration.
Underground filters are difficult to cause to function properly and
are typically utilized to maintain tank temperatures in the substrate for plant growth.
As Tracy partially indicated you may now have decaying matter in the UGF which is partially causing the high ammonia concentration.
I have a wet/dry system, am not familiar with cannister or HOB filters and therefore cannot recommend a filter for adequate filtration.
2) Water Changes
You indicated that you had not performed a water change.
Typically during cycling a 25% weekly WC IMHO would be appropriate based on the fish in and volume of your tank.
Refer to 3) for WC recommendations.
3) High Ammonia Concentration
As previously set forth fish will not adapt to a high ammonia concentration.
In the worst condition they will die.
In the best condition they will suffer permanent gill damage as well as damage to internal organs.
IMHO I would perform a 50% water change today and a 50% water change on Tuesday: then return to a typical tank cycling* subsequent to installing adequate biological and mechanical filtration (hopefully tomorrow).
Previously set forth in this thread was the concept that fish will become accustomed to high nitrate concentrations.
IMHO this is incorrect.
Nitrate concentrations should be maintained at 20 and preferably 15 or less.
*Please search this forum for utilizing media, gravel, etc. which is currently in a cycled tank in order to accelerate the cycling process in your tank.