Medications for Internal Parasites
Internal parasites can include Nematodes(Capillaria), Flagellates (Hexamita and Spironucleus)(probably the most common of the internal parasites in African Fish, this may be the cause of bloat/float), or tapeworm.
Treatment for each is with different medications.
Turn off UV lights when treating as it will affect the medications.
Remove carbon before using medications.
Medications depend a lot on the characteristics of your water (GH, PH, Bioload and results may differ from one tank to another!)

Levamisole Hydrochloride is used to treat fish that are infected with nematodes, an internal parasite. (Capillaria is one nematode)
I like to use Levamisole Hydrochloride in the water. Remove carbon from the filter before using this med. I have a digital gram scale and use this to measure this med at 8 mg/gallon. For example in a 120 gallon tank, I would use 8mg (.008 grams)times 120 gallons = .96 grams. I keep this “in the water for 24 hrs” and then do a large water change to remove the med. I repeat this in 1 to 2 wks. This med seems to do 3 things. Carbon can also be used to remove the med from the water.
1. it suppresses the appetite when in the water, so I either don't feed the fish or feed very lightly.
2. It is known as an Imunno-stimulator (it stimulates the immune system in the fish) (according to Dr. Gerald Bassler in "Colorguide of tropical fish diseases" book) What I notice after the wc is the fish are much more active and vibrant and one time my angelfish spawned immediately after using it and doing the wc. They hadn't spawned in quite some time. I buy this med at a local farm store. It is used as a pig dewormer.
3. It causes the worms to start to die and the fish to expell them in the feces.

I purchase this medication at a Farm Supply Store.

Metronidazole (Metro)
Metro is mainly used as an antiflagellate in fish intestines and organs. It kills flagellates. Flagellates are single celled protozoa.( most are oval shaped) They move by a small "whip like" organ called a "flagella".(some have one flagella, some have multiple flagella) In treating fish, Metro is mainly effective against "Hexamita, Spironucleus spp. , Trichomonas spp, Protoopalina spp."( all intestinal flagellates) (protoopalina is often confused with a ciliate, it is not proven to be a real parasite to fish) (Untergasser)
Metro does not affect nematodes (worms). It does have some effect on Ich, and on Cryptocaryon (marine ich).
Metro does have a "very limited" antibiotic effect. (kill bacteria)Mainly bacteria named "Campylobacter fetus"bacteria, and "Corynebacterium vaginalis" . (Untergasser)
It has some limited effect on some other bacteria depending on concentration. It is less active against non spore forming, gram + bacilli and even less active against gram + cocci.
(Untergasser, Handbook of fish diseases)
Now , there are some flagellated bacteria ("Microbiology in Patient Care" by Morello, Mizer, Wilson) Efectiveness of Metro is limited to very specific bacteria.
Metro kills the organisms by "disrupting the DNA of the organism after it enters the cell". ("The Pill Book" by Bantam Books, Silverman,Simon)

My point: Metro is mainly used as an antiflagellate in fish intestines, not as an antibiotic when using for fish.

Xtra info: There are flagellates of the fishes blood, skin, gills, and other organs. However Metro is not listed as effective against these flagellates according to Untergasser.
Signs of flagellates are generally not eating, facing the back of the tank and white slimy feces.
I usually use Metronidazole at 250 mg/10 gallons added each day for 5 days. I have used it at higher doses and for longer periods of time, but at the risk of causing kidney disease in my fish. It is a personal call as to whether to continue treating (is the treatment worse than the parasite?)
** I only use Metro when necessary, i.e. when there is white slimy feces. I don't use it prophylactically.

Praziquantel is very effective against tapeworms. It is dosed at 100 mg/10 gallons (1/4 teaspoon/20 gallons) for 24 hours. It is then removed from the water with water changes. IF there were tapeworms expelled, the treatment is repeated in 1 to 2 weeks.
Praziquantel may be effective against flukes.(external parasite) Up to 21 days may be needed to eradicate flukes as fluke eggs are not affected by praziquantel. Only when the eggs hatch is the medication effective. Some forms of flukes have developed resistance to the drug.
Prazi Pro may be used as directed for dosage and duration. For flukes it is used per instructions, for tapeworm it should work within 24 hours.
For Flukes, Praziquantel (pure) can be used at 100mg/10 gallons = ¼ tsp per 20 gallons. Water changes are done daily. If you change 20% water in the tank, then add back 20% of the total dose to maintain the “therapeutic level” in the tank.
Fish may lose their apetite when using this medication.
***Some fish have shown a “sensitivity” to this medication and have died.

Clout Also used for Bloat
Used as per instructions. It is effective against a wide range of parasites including Hexamita which may cause bloat. This chemical may suppress the apetite. Some fish may not tolerate this medication.

Epsom Salt for Bloating
This helps as a laxative. I use it at 2 tblsp/10 gal for 2 days. I use it at first signs of bloating/swelling of the abdomen. If the bloating is not alleviated in 2 days then it is probably an internal bacterial problem. This can be caused by parasites dying in the intestine and causing a blockage. Then an antibiotic must be used. My favorite is Kanamycin. Others have also had good results using Maracyn 2 (minocycline).

** IF I suspect a fish has internal parasites, I have used Levamisole, then waited 2 weeks and used prazi. I always give the fish a break between medications so as not to over stress them.

Here is a picture of the Hexamita Flagellate under the microscope. Used with permission from Handbook of Fish Diseases by TFH Publications

Here is a picture of one of my discus with white jelly like feces indicating internal parasite Hexamita or Spironucleus