Swollen/bloated fishes

Looks just like they are full of eggs

henningc said:

Looks just like they are full of eggs

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But the first one is of a male guppy, and as we know its the females that get pregnant (except seahorses of course).

Could be some form of ascites (commonly known in the aquarium trade as dropsy)
from googling:

The symptoms of dropsy in fish is a distended or swollen body with protruding scales. Dropsy is also known as ascites. The affected fish may appear lethargic and may not eat.
Dropsy isn't a disease itself, but actually a symptom of another underlying disease in the fish.The reason the fish's body swells is due to a build up of fluid in its body cavity and internal organs. This may occur if the fish's kidneys are affected, for example. There can be many different causes of fish dropsy, and it is usually difficult to know exactly what the cause is.
However, one common cause of dropsy in fish is a bacterial infection byAeromonas. Aeromonas is normally present in all aquarium water, however, it sometimes causes illness in fish, especially fish that are stressed due to poor water quality from overcrowding or infrequent partial water changes. Also, some species of Aeromonas are more pathogenic than others. Infection byAeromonas will sometimes cause red streaks or sores on the fish's body (but not always).

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Treatment:

You can treat the fish by adding a small amount of epsom salts to the tank. Epsom salts consist of magnesium sulfate as opposed to sodium chloride found in regular aquarium salt (and table salt). Adding epsom salts to your tank will help to draw some of the excess water out of the fish's body cavity and tissues. Don't add more than 2.5 teaspoons of epsom salts per 10 gallons of water.
You should feed the affected fish antibacterial fish food for 7-10 days if the fish is still eating. You can also add some Maracyn Two to the aquarium that the fish with dropsy is in. Maracyn Two treats gram negative bacterial infections, such as Aeromonas, and is absorbed through the fish's skin from the water.

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Source

EDIT: If you do end up using actual fish medicine be careful to read the instruction. Invertebrates and scaleless fish such as cories are highly sensitive to certain chemicals in fish medicines (and to copper as well in treatments like methylene blue). I'd go with the epsom salts and antibiotic food.

Hello! They most certainly could be pregnant. The fish in the first photo does look like a male, but it could just be a female with her anal fin folded up for some reason. If all of them are bloated, it could be overfeeding or an illness. I would maybe cut down on their feeding a teensy bit. If their bloating doesn't get better, keep a very close eye on them.
Dropsy is a very serious illness because it is difficult to treat. When a fish has dropsy, its scales start to "pinecone".

I would just keep a very close eye on them, maybe cut down on the food a little. Your white cloud minnows look like they are ok which is good!!
Have you added any new fish recently? Someone could've brought a nasty little bug into the aquarium.
Also a tip:
Keep the water clean if you think that it may be an illness. Regular water changes. If it is bloating, like said above, use small doses of epsom salts.

I would follow the advice of keeping clean water, observing them, and feeding less. Unnecessary medicating can do more harm than good. Also, from the pictures it looks like many of other fish also have pretty full bellies, so I'd guess it's more of an overfeeding issue.

I agree. Reduce your feeding. As you are new to fish, I'll mention that it is better to feed sparingly. Fish will usually eat if food is available, well beyond the amount they need. This is not good for the fish, any more than for us when we overeat. But with fish it takes considerable energy away from important functions when they are digesting so much food. Feed once a day, sparingly (only fry need more feedings), and miss a day or two; water change day is a good one to miss, as it is best to do tank maintenance in the early part of the day so the fish have a chance to settled down during the day, and you should not feed fish prior to maintenance.

Byron.