You have heard correctly.
People who eat some fish regularly are sharper and less depressed.
Unfortunately fish are becoming both costly and they are disappearing, like cod (one of my favorites).
If you cannot afford fish more often, you CAN afford fish oil capsules, a daily one. An occasional nip of cod liver oil was a staple from grandmother-of-older-answerer's point of view. Still available, anyone can buy.
The B-vitamins are a part of most daily vitamin supplements.
The one assisting depression most is B6. But there is an analogous one that works more directly and reliably for depression:
It is called SAMe. s-adenosyl methionine, something like that, thus the abbreviation. For depression you take 400 mgm twice daily. After a month that can be cut back to 400 mgm once daily.
SAMe is NATURAL. The prescription remedies are UNNATURAL and have SIDE EFFECTS, often dangerous. Especially in combination with other unnatural drugs.
So it is good to stick to fish oil, SAMe and natural things which have no dangerous side effects.
Only drawback about SAMe is that it is somewhat pricey for many people. If you cannot afford it, best alternative is to take additional Vitamin B6 along with your regular daily multivitamin.
Other things that work for depression are:
FINDING GOD (my blog or Q&A helpful)
HOBBY with FRIENDS
WORKING at something you like
WORKING with FRIENDS
In fact, the most enjoyable of all times for me has been working with people whom I respect, value and like.
The worst has been working in other places where there are no such people. But I got through a lot of that, having God.
God did not create man to be alone. We all need others in our lives.
In some way. For an old man like me, I am satisfied to help others online.
But you are too young for that to suffice. You will need and will profit from a few good friends.
Become interested in something worthwhile and join a group. Mechanics, woodworking, bird houses, veterinarian assistant, whatever.
Girls seem to be better at this, but as a former medical worker I can also attest they have worse problems with depression.
Men get depressed in work situations where there are no women around (also know from solid experience). I do not know about vice versa, but suspect that may also be true, ask a female prisoner.
So men and women need each other, not just for sex.
Men and women are complementary, like love and wisdom.
So if your hobby or group involves both, that will be significantly better.
But you will improve by finding out more about God in a direct relationship.
My blog and Q&A show how to establish a direct relationship, results from God -- not from me. You find out for yourself that way.
And it is REAL, not just being a follower of some minister or just a member of some church.
That is what God wants: A real relationship with you as your Heavenly Father, also through Christ and The Holy Spirit.
You can delve in my Q&A or blog, but you will need to really dive in and work about there.
All in one place, what else could you ask for?
BTW, I did lots of medical work so you can rely on the natural remedies part.
Found God a necessary part of natural remedy while doing years of medical work.
Often the key to really helping, the power by which everything else works at all. Drink more water, too!