You may consult any of the following services:
[Many of them assume/consider Tether an Omni rather ERC20]
Please make sure to check each one's pros & cons against others, Then stick with what you've selected. Good luck!
The most secure way is running your own full nodes per Blockchain.
Using third-party services such as blockcypher will put you & your business at high risk because of:
You're giving the keys to your home & revealing the password of your bank account to someone else! Because in order to use such services, they are the one who make the public & ...
Here is one way to do it: https://github.com/nateGeorge/bitfinex-ohlc-import
Also check out the original repo I forked from. Basically I get the earliest candles, then work up from there. If there are some candles in the DB already, it updates to the current time.