In Terraria, the biome is determined by counting up blocks 50 tiles to the left and 50 to the right and 43 tiles up and 43 down from the player (that's a 100 x 86 tile rectangle, centered around the player). If enough blocks of a certain type are found in that area, the biome becomes that type, although there are a couple of other rules that also apply.
The counts are this:
300 blocks of Ice (of any type) or Snow = Snow biome
200 blocks of Ebonstone, Ebonsand or Purple Ice = Corruption biome
200 blocks of Crimstone, Crimsand or Red Ice = Crimson biome
100 blocks of Pearlstone, Pearlsand or Pink Ice = Hallowed biome
80 blocks of Jungle Grass or Lizhard Brick = Jungle biome
100 blocks (200 for fishing) of Mushroom Grass + possibly Glowing Mushrooms (not sure about this) = Mushroom biome
There are also other rules that go into this:
Hallowed blocks and Corruption/Crimson blocks of any sort mutually cancel out each other with 1 Hallowed canceling 2 Corruption/Crimson.
If there is more than one biome that can be in effect, the biome with the highest priority takes affect. The priority order for fishing is (copied from the Wiki):
Lava > Honey > Corruption = Crimson > Hallow > Snow > Jungle > Mushroom > Ocean > Forest
The complex takes all three of these factors into account. Since Hallowed and Corruption/Crimson cancel each other out, the Pearlstone (Hallowed) is on one side and the Crimstone (Crimson) / Ebonstone (Corruption) blocks are more than 100 blocks away on the other side. The blocks determining the biome are also arranged so that the required number comes into range at the right point as you move across the pool and that biome is overridden by a higher priority one as the new biome's required blocks all come into range.
There is some oddness in this in the music playing and background / water color for the biomes. This is because of the priority order for music, etc., is different from that of fishing. This order is (again from the Wiki):
Ocean > Mushroom > Corruption > Crimson > Jungle > Snow > Hallow > Others
This order mismatch has the odd effect of playing Jungle music while catching Snow fish.
Another little oddness is that because Crimson and Corruption are equivalent in the priority order for fishing, moving from Crimson into Corruption doesn't cause Crimson to be overridden and so you will get both Crimson and Corruption catches in the Corruption fishing spot. The music priority order, however, is such that you will see the water change color and the "sky" background change in the Corruption fishing spot.
The optional Mushroom biome is a bit of a problem because of this different ordering, which is why the Mushroom Grass blocks are above the complex rather than to the right with the other biome blocks. Mushroom biome fishing is done by jumping up 6 tiles and coming into 43 tile range vertically of the 200 Mushroom Grass blocks rather than moving to the right and left on the upper pool. This vertical placement stops the Mushroom biome music/background from always overriding that of the real biome used for fishing.
Other than what I think is a unique placement of biome blocks, I didn't figure out any of this on my own. Much of the information came from the Wiki article on fishing but there were two other major sources. First, a thread started by
@Danke here,
http://forums.terraria.org/index.php?threads/project-ultimate-fishing-semi-autofarm.6864/, which covered many of the fundamentals of fishing biomes (although, curiously, it had the player's height as being the determining factor for the fishing level rather than the real determining factor, the height of the fishing bobber) and I owe much to the thread's many contributors. The second was a link pointed out by
@inomanoms and made by "pakratt" here,
http://imgur.com/gallery/DapM6, that had a 1-Tile Fishing Hole, and this link started me thinking about compacting the build.
