Tank EVERYTHING. Be a cooldown sponge. You just want to get beefy and team fight.

When the fight breaks out (or you start it by ulting the main enemy carry), jump right into the middle of the enemy team and try get the attention of their carries. Get as low as possible then jump out of the fight. You never want to die, however the lower health you get out on the better you've done... Because that means you've blown as many enemy CDs as possible. Once you're out, try go back in again. You are never too low. Make them want you. Make them focus you. They will die trying.

Pros:
-Unkillable, so you can blow insane amounts of enemy cooldowns and still survive. Your team will then clean up the fight.
-Impossible to peel off the enemy carries.
-Still does decent damage due to cinder, percentage damage from W and thornmail.
-Extremely easy to turret dive with.

Cons:
-Less crowd control than many tank junglers.
-Weak ganks due to no burst damage and the lack of early crowd control.

The only big problem you have is the weakness of your pre-6 ganks.

Solution 1: Farm to 6.

Solution 2:
Communicate the circumstances you need to pull off a pre-6 gank. Because you are Fizz people will think that your ganks are much more bursty than they actually are. The way you have to gank pre-6 is wait for the enemy lane to push into you, then get your team mates to engage first. Once the enemy laners have used their CDs jumping in, you can come in, apply your slow and chip them down to dead before they can get back to the safety of their turret. If your team mates don't understand this, don't gank their lane... Just go elsewhere or keep farming to 6.

Another situation where you can gank is when the laners are very low and you can turret dive. In this situation, make sure you communicate that you are diving, wait until the wave is pushed in, take turret aggro first then E out at as low health as possible.

Once you hit 6 your ganks become quite strong. The best thing to do is look to turret dive a low health enemy as soon as you have your ult ready. Remember to communicate this.

0 30 0. Sure 9 21 0 would be more standard... But that's not in the spirit of things! You can still clear just fine with 0 30 0 and it will make you more tanky... And tanky is good. Very good.

You want as many scaling tank stats as possible, in order to be unkillable mid-late game. As you can't get scaling marks, get flat armour, which will also help with your initial clear. Take 5% CDR glyphs so you cap out your CDR at exactly 40% when you have your CDR items. CDR is just as important as tank stats on 0 30 0 Fizz.

W is your damage skill. You have no AP or AD so your damage all comes from your % W damage. Max this first. W is also the best skill for jungling.

E second for more jukes, plays and general survival.

Q last as it is only really a utility spell on 0 30 0 Fizz.

R whenever possible. It's damage is very poor... but the slow, knock up and zoning potential are really nice.