Hi Debbie. Welcome to ATP!
No one has said it yet, so I feel obligated. In theory at least, the only way to KNOW what your soil needs is to send some out to be tested. You MIGHT already have so much nitrogen and organic matter that you don;lt need much compost added.
But who sends their soil out to be tested??
Growing in a container, even a large container, is rather intensive compared to growing in soil.
You might have exhausted the relatively small amount of soil in the tank, and need to replace it all, or add 50% compost plus drainage amendments.
You might have made such rich soil last year that you would be better off flushing it out thoroughly, than adding more rich ingredients. But it might still benefit from restored drainage.
It's hard to guess, with containers.
My rule of thumb is that if the plants were not yellowing and stalling their growth last year, they had "enough" fertilizer or organic matter.
Too much fertilizer is worse than not enough, so I go easy rather than heavy when adding fertilizer (chemical fertilizer). If you are all-organic, you aren't likely to over-fertilize but might still have richer soil than you need.
With huge amounts of compost, I MIGHT ask about residual herbicides, weed seeds and excessive salt, but it sounds like ARC Gateway knows to avoid such pitfalls. What a great find!
I would say to add as much compost as you need to keep fertility and water retention moderately high - but how much is that? Probably grabbing a double-handful and guessing is the only way.
One thing can be said: you need drainage and aeration, ESPECIALLY in a large container with only one drain!
if the soil has become AT ALL heavy or dense, or tends to get water-logged AT ALL, or if the soil becomes too heavy before water runs out the bottom -
Improve your drainage and aeration.
Maybe I'm a nut about that, but I bet lots more container plants die from drowned roots than all other causes combined.
Now, while nothing is growing in the tank, is your only chance to amend the soil. Even if you have to shovel half of it out so you can amend the bottom half (the most important half), DO IT.
Would your tank benefit from improved drainage? I can't know because I can't push my hands into it, or watch water come rushing out the bottom seconds after I pour a few gallons in the top.
Imagine you were a college freshman locked into a telephone booth. That's like a plant with its roots in a container that drains slowly because the soil mix is fine and air channels have become clogged or were small to start with.
Now fill the phone booth to the roof with beer, and water the plant so the soil is nice and moist on the surface, damp below the surface, and drowned in the bottom 2/3 of the pot.
The freshman drinks the beer as fast as he can, hoping to create an air space to breath in before he drowns.
At the same time, the plant ... can't do anything. Its roots are underwater, and oxygen diffuses very SLOWLY through soil filled with water. It's like a kid chained to the very bottom of a swimming pool- if the air doesn't come to him, and he doesn't have a good set of gills, it's going to be bad for the kid.
The freshman drowns and the roots rot because they can't get their space free of water fast enoguh to prevent suffocation.
Running out of water and wilting is nothing. Add water - recover - all better.
Roots drowning or going hypoxic is bad: the roots rot and die. Maybe they try to grow back between waterings: then the die and rot again.
Fast drainage is a Good Thing.
If you pour water in and nothing comes out, that's because the water displaced air in the voids and channels in the soil mix. that doesn't sound too bad until, you remember that roots were getting their air through those voids and channels, like sipping air through a straw while hiding underwater. If the straw is too skinny, or fills with water, you don;6 get enough air.
When you dig into your tank, do you see lots of big, white healthy roots all the way down to the bottom?
If not, they were probably drowning and dying back after heavy waterings.
Or maybe not - I am obsessed with drainage!
(I guess a really big, deep tank with few plants would not fill to the bottom with roots because the plants had more soil than they needed. I've just never seen a container or raised bed like that!)
How much drainage-enhancer to adds depends on how well your soil drained last year, and how much it broke down over the course of the year. Anything organic breaks down, and the fines and colloids plug up the air gaps in soil and turn it into a root-drowning medium.
But you should be able to water and see water come out the bottom pretty quickly!
If you have to water it until the bottom is heavy mud, before water pours freely out the drain, it could benefit from improved drainage.
Grit is great.
Crushed rock is great.
Coarse Perlite is great (but perhaps expensive).
Very, very coarse sand is good. (These first four are permanent. They will improve drainage for centuries.)
Screened pine bark is great: 1/10" is a good size, but smaller bark fibers could help clay-ey soil.
But it breaks down over several years.
Coarse coir is OK, but how long does it last?
Peat moss is much too fine and breaks down much too fast.
Maybe [u]Sphagnum[/b] peat is better than fine brown peaty-dust, but I have yet to buy a bag of peat that did anything but hold too much water and then crumble into dust.
Vermiculite is bad: it crumbles at a touch and becomes fine powder that will sift into air channels and plug them tight. Avoid it.
All just my opinion. Plenty of people grow in containers without being ultra-paranoid about drainage.
But my theory is that they lucked into, or carefully chose, soil mixes that drain very well even after a few years of use. if they hadn't, they would be wondering why nothing grows any more.
Welcome to ATP.
Sorry, I just couldn't resist. 
totally, that she didn't give us enough info initially, but I do think we 'muddied the water' a bit, too.
