There are many different paths you can take when leveling up to Tier 10. Here are some of the fastest and most effective:
Lampreys
You can stay as a lamprey, attach to a max rank host, and afk farm food that way. This is one of the safest strats but may take longer than others. Make sure your host is welcoming to lampreys, since lamprey's low oxygen time allows any host to easily suffocate it by going out of water. Remember not to level up until you have enough xp to cruise straight to rank 10.
Piranhas
One of the most popular and fastest ways to evolve is to use the animal Piranha. Just keep boosting and leveling up on the Piranha tree until you get to tier 10. Try killing sardines and small animals for quick and easy XP. For low risk and reward, don't evolve past tier 6 as you will not be able to eat algae (ground food.) Otherwise, evolve to T7 to swarm an animal you can kill, and evolve into any tier 10s.
Frogfish
This is the most popular and fastest way to evolve. Upon reaching tier 7, choose frogfish and use your suck ability to farm food. This goes even faster if you use piranhas tiers 1 to 6.
Cookiecutter and Penguin
This can be a very quick evo sometimes, but there is some risk of having your food stolen. Spawn as icefish so you spawn in the arctic, then look around for a lion's mane jellyfish (stay a tier 5 seal until you find one). Evolve into cookiecutter shark and continuously latch onto the jellyfish. The bleed will eventually kill it. Be sure to kill it before your pressure time runs out. Once you kill the jellyfish, farm for a bit as penguin before evolving to the final tiers.
Catfish
Find regions with a lot of plankton, and suck in them using your ability. Great in farming XP but you are vulnerable to most animals, so staying close to broken trees is a good idea.
Bobbit Worm
A safe but slow method. However, remember you will now lose salinity in the swamp. A extremely fast way to get xp is to sweep the icebergs at the bottom of the arctic because plankton spawn instead of algae. (Click here to go back to tips)