Returning to Feed the Fish Before Eating

I went out to eat at a restaurant, and on the way, I remembered that I hadn't fed the fish in the aquarium. What should I do?

You should return to feed the fish, or you can ask a neighbor to come in and feed the fish. Once you have asked the neighbor, you can begin eating and do not need to wait for them to actually feed the fish.

The Gemara in Berakhot, page 40: "Rav said, 'Take the blessing, take the blessing' — one does not need to bless. 'Bring salt, bring a condiment' — one needs to bless. Rabbi Yochanan said, even if they brought salt or a condiment, one does not need to bless. 'Feed for the oxen, feed for the oxen' — one needs to bless. Rav Sheshet said, even 'feed for the oxen' does not need a blessing, as Rav Yehuda said in the name of Rav: 'It is forbidden for a person to eat before he gives food to his animal,' as it is stated: 'And I will give grass in your field for your cattle, and then you shall eat and be satisfied.' The Gemara discusses a person who blessed 'hamotzi lechem min haaretz' and then spoke before eating, and he must bless again unless the speech was for the purpose of eating. The opinion of Rav Sheshet, and so ruled the Shulchan Aruch in Orach Chaim, siman 167, is that even if he commanded his household to feed his animal between the blessing of 'hamotzi' and eating the bread, he does not need to bless again because it is forbidden for him to eat before he feeds his animal, and it is considered necessary for eating. In Biur Halacha, siman 167, 'and nevertheless,' the opinion of the Magen Avraham is brought that the prohibition to eat before the animal is from the Torah, but the Baal HaMaor and Sefer HaEshkol hold that this law is rabbinic. And what is written that when he commanded his neighbor, he can eat and does not need to wait, this is proven from the words of the Gemara, for he commands his household to feed, and why not feed himself and interrupt with silence without speech, and this is a lesser interruption, for in a case of necessity he does not need to bless again even if he was silent more than the time of speech. This proves that by commanding, he can eat immediately after the command, and therefore the speech itself is considered necessary for eating.