My Main Deck

Firstly, Figuree out your (usually) 1-4 bse cards. These cards are the most important for you to win and should always be 4 copies of. (legendaries should only be 2, as otherwise you'll most likely end up with 2-3 of them on hand a lot of the games)
After this, figure out what cards can be there to cover their weaknesses or build more on their strenghts to ensure that you'll win. You should always have a backup plan in case the cards you use to win is somehow countered in some way (like how a mycoloth deck would need some way to boost toughness to make sure that they don't die to a simple "Illness in the Ranks")

Now. What are your 1-4 base cards? I am guessing Talrarnd is one of them as he is on your decks picture. In that case, building a lot of Instants and sorceries and ways to get more of them is a good idea. Start with making sure you have 2 Talrands, but also that you have more cards that works very well with instants and sorceries. "Guttersnipe" would make a great addition as you won't always end up with Talrand in hand.

Cards that I really like in this deck: Augur of Bolas, Essence Scatter, Negate, Rewind, Searing Spear, Unsummon, Talrand's Invocation, Divination, Fog Bank, Archeomancer, Sleep.
These cards should be 4 of IMO, some of them could be 2 of just because of the low number of times that they're actually usefull for you (like sleep!)

Cards that I don't like in this deck:
Elixir of Immortality (why would you want to remove cards from your yard? The life gain is not that great either. You have archeomancers for taking back cards)

Ring o Evos Isle (if you're burning down the enemies creatures and countering stuff with counterspells, why would you need more countersr on your creatures? I don't see how an army of dragons and/or a massive amount of damage from guttersnipe could go wrong)

Harbor Serpents (there are way better cards than this)

Kraken Hatchling (I really think Fog Bank is just better than this. You'll rarely use this for anything but blocking. Everyone has their own taste in defensive creatures I guess)

Mindclaw Shaman (it's completely ok, but it really belongs in the sideboard imo, because not everyone plays with that good instants and/or sorceries)

Scroll Thief (this one is also completely ok, I can see why you'd want to draw more cards, but there's a card called "Cold-Eyed Selkie" that nets you cards vs blue easier and can also net you more cards depending on how strong it is. It's not cheap though, I for one only bought 2 for my merfolk deck! Could be something for your "Ring of Evos Isle" to make it just a bit better though lol)

Stormtide Leviathan (It's ok, but it won't always help you and can sometimes just be something that sits in your hand for the entire game. I for one would switch it for something more useful)

Wall of Fire (it's a wall. It helps defend and it can actually kill things off. It's ok, but I personally just don't like it over Fog Bank)

Wind Drake (this card is nothing but "ok". It is not any stronger than one of the dragons that you get from "Talrand's Invocation" and that card just costs 1 more mana to cast! One more mana for one more of those drakes is definetly worth it!)

Omniscience (This is like stormtide Leviathan. Also, what are you going to spend your mana on IF you get it in play and IF you still have things to cast with it in play? I'd change it for a more useful card.)

Pursuit of flight (this card gives things more p/t and flying. It's an "ok" card, but doesn't work with that much stuff in your deck.)

Hydrosurge (imo, there's better ways to stop an attack, like "Azorius Charm" (that card is AMAZING btw, I'd suggest it for your deck!) it's another one of those "ok" cards.)

Smelt (everyone is not going to play with artifacts. This belongs in the sideboard)

Switcheroo (you are the one trading the dragon here, not them. This is "ok" but only as long as you're the one trading the turtle (this means that you're the one usually trading the better card and they are trading a worse card, as you've probably Essence Scattered all their good creatures into oblivion, or thrown a couple of spears at 'em.))

Turn to Slag (again, equipments is not something everyone plays with. It costs a lot of mana too and frankly, there's better burn spells than this.)

Lastly, I hope you don't think I'm trying to be rude here, I'm just letting you know what I think of the deck! I now realise that this is a wall of text to read, but you can decide yourself if it is worth to read it or not. If you reply with information on how you change the deck, be sure to know that I will reply to help you further!
Have a good day, sir! :)