In the U.S., a conversion from one token or coin to another is a taxable event. You must report the gain or loss in value of the coin you converted from. Unless you transact in huge amounts, the gains or losses with stablecoins might be too small to report. I think the threshold is $10.
If anyone from the UK is reading this thread, it's the same here, too. Converting, for example, some Bitcoin into Ethereum is a taxable event, based on the price as at the moment of conversion. If you keep your trades below the capital gains threshold, you should be okay... but any gains over that need to be reported, as I think do trading volumes above a certain threshold, irrespective of whether or not you profit.
I am not a tax expert.
Me neither. There are plenty of qualified accountants who are happy to provide advice on crypto. By all means listen to random forum posters like me, but if you want proper advice then go to the experts.