Or it could be that their diet is coming up short on nutrition. 

Or both the above.

The foods you're feeding aren't adequate for a long term healthy life.  More on that here:

http://www.reefland.com/forum/marine-fish-care-health-disease-treatment/18767-feeding-marine-fish-fish-nutrition.html

Lee

I agree with Lee - intestinal worms is always a possibility.

Do you enrich their diets with any supplements?

What type of substrate is in your tank?

Just because a fish eats readily what is offered doesn't guarantee the fish will thrive or survive long term.  The diet must meet the fish's dietary requirements.  The diet for valenciennea gobies should consist of meaty foods that are vitamin enriched - crustaceans, frozen enriched brine shrimp, mysis, blackworms, glassworms, finely chopped squid, shrimp as well as the commercially prepared foods for carnivores just to name a few.  They need to be fed several times a day. According to Scott Michael these fish are often difficult to feed/keep and "many slowly starve to death"  He suggests to improve long term survivability that they need to be kept in a tank that has live sand and a well established refugium.  Scott (along with I ) strongly suggest deworming your gobies (and all fish). 

An invaluable reference book (inexpensive as well )on husbandry needs is Scott Michael's Marine Fishes.

Best of luck.

I've posted it on Reefland.com as a sticky.

Good luck! 

Lee

I recommend praziquantel as my first choice of dewormers.  I also prefer administering in the food versus adding it to the tank.  The dose for praziquantel is 23 mg per pound of fish weight. (Noga - Fish Disease)  Problem I see with you ordering the praziquantel on line is the volume that it comes in and your gobies weigh probably an ounce a piece.

There is a commercially prepared anti-parasitic pellet food available now.  It is made by Jungle.  The active ingredients in the  anti-parasitic food are metrodinazole, praziquantel & levamisole.   This may be a good choice for your small fish.  Unfortunately with my large predators, this is not an option

I would follow the directions on the bottle and ONLY feed the medicated food during the treatment period.  After the treatment, I would focus on improving the diet.

What type of substrate do you have?  Is there a refugium or live rock in this tank?

Best of luck.

Formula 1 Flake

New Life Spectrum pellets

Algae Wafers

Shrimp pellets

Frozen Saltwater Mysis

Frozen Cyclopeeze

Dried Marine Algae sheets

One of the gobies looks exceptionally healthy with long fins, bright colors, and a plump round belly.  It would seem like if nutrition was the issue both gobies would be in poor health.  The skinny goby does not appear to be sick except that he is exceptionally thin.  He is always out with his mate, constantly sifting the sand, and will fight with other fish(even his mate) on the sand bed or midwater for any nearby food.

The substrate and the refugium are great!

I definitely would deworm these guys.  Just because one is fat, doesn't mean it doesn't or can't have worms......just the worm count hasn't increased to a number that the dietary intake can't keep up with.

The recommended diet for gobies is meaty - I would tend to give more meaty foods & less algae wafers, herbivore food.

Do you enrich your food?

To answer your question, no I do not currently enrich any of the foods that I use.  What should I be adding and can I pick it up locally?