Unique shipping rules and quantity discount advice needed

I sell live fish.

The base price for one type of fish is $1/each. I require the customer to purchase a minimum of 25 fish, but offer the option of buying more in increments of 1 fish if they would like, depending on the number I have in stock of course. I ship 25-100 fish in the smallest shipping box, and the shipping weight will stay the same at 5 pounds since it is the water that the fish are in that actually causes the weight, not actually the fish. In other words, if I ship 25 fish or 100, the shipping cost will be the same. However, if they purchase more than 100 fish, I have to use the medium shipping box, which is 10 pounds in weight. I can ship 101-250 fish in this medium box. If the customer purchases 251-500 fish, they ship in the large box that weighs 25 pounds. Additionally, I offer quantity discounts.

Examples:
25 fish = $25 + small box shipping
27 fish = $27 + small box shipping
150 fish = $150 - 10% discount + medium box shipping
350 fish = $350 - 20% discount + large box shipping

How would I go about setting this up in zen cart? I ship my fish via UPS, and I currently use the Ceon Advanced Shipping module since I also have a few drop shippers (who incidentally follow the same shipping rules as I do). As a work-around to my problem, I have set it up so that 25 fish is a product, 50 fish is a product, 150 fish is a product, etc. However, this means I am unable to use the built in stock quantity tracking system which is a major drawback, and I also am unable to allow customers to purchase fish in single increments (can't buy 27 fish if you want to for example).

Ideas, thoughts? Am I asking too much from Zen-Cart?

I haven't used the Ceon module, but the Table Rate would work for the basic stuff you need, using the number of items as the criterion. When you configure the Table Rate module, there are some basic instructions.

The quantity discount would be separate from the shipping.

Thanks for the quick reply Steve. I actually just figure out a way to make this work using the rate table option of the ceon advanced shipper module. What a relief! This will make operations a LOT easier!