The more you start to learn about fish, the more there is to know!  There are hundreds of important terms and words -  it's literally a whole other world.  Here are just a few words and definitions that I come into contact with regularly -

acidic - Water with a pH of less than 7.

adipose fin - The small fin located between the dorsal fin and the caudal fin. It seems to serve no purpose.

 alkaline - Water with a pH between 7 and 14. Also known as Basic.

ammonia (NH3) - A dissolved gas that even in low concentrations is toxic to fish. It is produced by the breakdown of organic waste products.

asexual reproduction - Asexual means having no sex or sex organs, therefore asexual reproduction would be reproducing by means other than sex.

amyloodinium - Amyloodinium ocellateum is commonly called Oodinium, Marine Velvet, or Saltwater Ick.  Possible signs of the disease are cloudy eyes, gasping for breath, listlessness, and white spots.  Positive signs of the disease are gold or brown spots, rough skin, and rubbing against rocks, etc.  Treatment can be done by a freshwater dip and copper (as long as inverts are not in the tank).

anal fin - Single fin mounted vertically below the fish.

carapace - A bony or hard shell that covers part or all of an animal. Turtles, crabs,  and boxfish are good examples.

caudal - The single fin mounted vertically at the rear of the fish. The tail fin.

caudal peduncle - The part of the body which attaches the caudal (tailfin) to the body.

color rendering index (CRI) - A number used for rating lite bulbs on a scale up to 100, where 100 is equal to sunlight. 

cryptocaryon - (fish pictured on left DOES NOT have Cryptocaryon)  Cryptocaryon irritans is a parasitic infection where white spots appear on the body and fins. Fish will scratch themselves against rocks and breathing may become rapid if gills are affected.

Treatment can be done by copper or other anti-parasite remedies, but this is incompatible with inverts. Cleaner shrimps and wrasses will remove the parasites, but may not keep up with a major infestation.

Cryptocaryon is often referred to as the marine equivalent of the freshwater white spot disease, Ichthyophthirius, or Ick.

cyanobacteria - Cyanobacteria is commonly referred to as red slime algae by hobbyists although it is not really an algae. It is a bacteria. Poor water quality with excessive nutrients are the usual causes.  To combat, do frequent water changes, siphon out detritus, and use a good protein skimmer.

denitrification - Breakdown of nitrates by anaerobic bacteria into other forms.

detritus - Grayish piles of organic compounds that accumulate in the aquarium. Commonly will contain fish wastes, fragments of rock, leftover food, among other things. Usually detritus will accumulate in low water flow areas, sumps, etc.

detrivores - Animals that eat detritus. Common detritvores are urchins, stars, hermits, etc.

dolomite - A limestone gravel with a small pH buffering ability.

dorsal fin - The fin directly on the top of the body (it's the fin that sticks out of the water when you see a shark). Some fish have two dorsal fins one directly behind the other.

filtration - Method of cleaning aquarium water, there are 3 basic types. "Mechanical" removes particulate material. Chemical" removal of dissolved substances by passing through a type of media, like carbon. "Biological" which is the process of changing from a harmful substance to a less harmful one, by bacteria.

fry - Fully formed baby fish.
 
hard water - Water with a high concentration of dissolved salts.

head and lateral line erosion - Also known as hole-in-head disease and lateral line disease. A fish with this condition will develop holes in its' head and sometimes along its' lateral line. The main cause is nutritional deficiency, especially vitamin C. Stress and poor water quality also play a role. Untreated cases will cause disfiguring or death. To combat and cure, ensure good water quality and provide vitamin enriched foods, especially vitamin C.

herbivore - Animals whose diet consists mainly of vegetable matter.

hydrometer - A device used to measure salinity of water.

ich - A very common parasitic disease characterized by white salt-like specks all over the fish. 

invertebrate - Commonly called inverts by many in the hobby. Invertebrates are animals without backbones like anemones, corals, shrimps, snails, and crabs.

macro-algae ~ Complex celled algae found in marine aquaria i.e.: Calurpa.

mbunas - Common name for rock dwelling African Cichlids from lake Malawi.

metal halide - A very intense type of lighting used in marine and freshwater plant aquaria.

micro-algae ~ Single celled algae often growing in strands called hair algae.

nematocysts - The cells at the tip of an anemones' (corals) tentacles which "sting" when touched.

nitrate - Mildly toxic end product of biological filtration. Can be used as a gauge to tell when a partial water change is needed.

nitrite - Middle element of the biological cycle.

nitrogen cycle - The formal name for the biological cycle in which toxins are broken down into less harmful products by Aerobic bacteria.

oodinium - Known as velvet disease, similar to ich but more deadly.

pectoral fins - The anterior or dorsalmost paired fins of fishes. They correspond to the anterior limbs of the higher vertebrates.

pharyngeal teeth (throat teeth) - Tooth-like structures in the back part of the throat derived from gill supports. Primarily used for grinding, may be used to produce sound.

pH - Measurement of the degree of water acidity. It is determined by the amount of acids and bases in the water. If the amounts are equal it is called neutral with a number rating of 7.0. If more acids it is called acidic with number rating of less than 7.0, if more base it is called alkaline with a number rating of more than 7.0.

phosphate - An element introduced into the aquarium through organic or inorganic means that promotes excessive algae growth. Can be removed through water changes.

photosynthetic - The process by which organisms, usually plants, use the energy contained in light, usually sunlight.

phytoplankton - Microscopic free-floating aquatic plants, mainly algae. It lives suspended in bodies of water and drifts about. See plankton.

plankton - Plankton are the drifters of the sea. Although they may have some form of locomotion they are mostly carried by water currents. Plankton is divided into macroplankton (jellyfish, sargassum weed) and microplankton, organisms that can only be seen by a microscope.  The microplankton is divided into zooplankton, tiny marine animals, and phytoplankton, or plants. Most fish start their lives as small animals in the plankton.

polyp - A polyp is the living unit of a coral.

predaceous - This means the animal will hunt and eat other animals. The old rule comes to mind, big fish eat little fish! 

red Slime - see cyanobacteria.

sponge filter - A large foam rubber block connected to an air lift tube. Water is drawn through the sponge, which acts as a mechanical filter and a bacteria colony. Especially useful in breeding or fry tanks.

substrate - The material used to cover the bottom of the fish tank.

trace elements - Minute substances found in water,that are needed for proper plant and fish growth. Usually replenished with partial water changes.

trickle filter - A wet dry filter using a drip plate to disperse the water over bio-media.

tritrate test method - A specific method of water testing in which a reagent is dropped into a sample until a color change is noticed. More accurate than color comparison test methods.

turnover rate - The number of times the water in an aquarium is passed through the filtration system in one hour. The minimum should be three or four times per hour.

under-gravel filter - A type of filter placed under the substrate that utilizes an air pump or power head to pull water through the gravel and thus oxygenates the bacteria to help in the nitrogen cycle. Not recommended for live plant tanks.

yellow water - Name given to water with a slight yellow tinge. Can be caused by too much organic material in the water or to show that a water change is needed. Eliminated through carbon filtration and better water management.

zeolites - A substance used in fresh water aquaria for the removal of ammonia.

zooplankton - Tiny animals or larvae of other animals living in water. usually used in reference to marine aquaria.

zooxanthellae - The symbiotic single celled brown or yellow algae living in some marine invertebrates. They supply important elements to their hosts.

This is just small sampling of words I hope you find helpful!  If you want to learn even more, stop by  ANIMAL-WORLD  -  it is a great resource  online full of all kinds of useful tropical fresh and saltwater fish and aquarium information!