On our way to Ben's wedding (Dee's son) in Plant City, Florida, we decided to have a small adventure: spontaneously stop, without plan and swim and snorkel at Silver Glen Springs in the Ocala National Forest (located between highway 19 and 40). Click here for all the interesting places you can visit, swim, camp and snorkel in Florida. This is actually preparation for another camping trip with Adi and Laura in Greece - and its essential to test camera, mask, snorkel and fins. The spring feeds a river that flows to the Atlantic. Consequently, there are several types of fish that swim upstream - from salt water the fresh water and can be found here.

Joseph Wolfersbers explained why:

My undergrad degree is in Biology and I took classes in Oceanography and 
Limnology at the University of North Florida in Jacksonville.  The water 
in the St Johns is brackish as far upstream as Palatka. The St Johns is a 
really interesting river it only drops in elevation less than 10 meters from 
its source to the ocean and that is over a 300 mile course.You are familiar 
with Salt Springs I am sure. The mineral content is rather high in Silver Glen 
as well. My professor at the time said that since the calcium carbonate level 
is so high the salt water fish can still osmoregulate.  

We stopped at the park entrance - and were the only folks there. Look at what we found:

(May 25, 2004) Bill Moody states that its a Nile Perch or Tilapia...It is not a native fish, but was imported for weed control since it is herbivorous and eats many of the weeds that choke Florida waterways. Many of these weeds are also not native to Florida. The Nile Perch seems to have a soft impact on native species in that it doesn't compete for food, but in fact supplies food to other species and is even pretty good fare for the "human" species. The only conflict I know of really is breeding (bedding) space.