Feed is needed to earn TDT. Each tweet consumes one feed, and once the feed is gone, you will not earn TDT by tweeting.
There are different levels of Fish NFT, and the amount of feed refilled depends on the level.
You can store up to 10 Feeds for each NFT. If you have 10 Feeds, the number of Feeds will not increase even if they are refilled. You can store up to 50 Feeds by purchasing a Feed box (available at the Shop).
As mentioned earlier, the higher the level of NFT, the bigger the amount of TDT eaned per tweet, so when combined with the increase in Feed refilled amount below, the effect of upgrading the level is significant.
*Note that Baby NFT is only level C and cannot be raised to a higher level.