can pest snails make bettas sick?

[chanliteheart]

Housing
What size is your tank? divided 15 gallon planted tank with 2 betta boys, Sapphire and Cobalt. (We keep out 2 gallons for a lower water level.)
What temperature is your tank? 78
Does your tank have a filter? Yes, Aqueon 25, I think
Does your tank have an air stone or other type of aeration? no
Is your tank heated? yes
What tank mates does your betta fish live with? Just the 2 boys & now a ton of pest snails.

Food
What type of food do you feed your betta fish? each boy 2 ZooMed pellets, 2x a day

Maintenance
How often do you perform a water change? Every other week,
What percentage of the water do you change? alternate 60% & 75%.
What type of additives do you add to the water when you perform a water change? Stress Coat (at 1ml per gallon since one jumped the divide, major fin damage to both.)

Water Parameters:
(this is the main tank, QT’s were 100% water change daily.)
Ammonia: 0
Nitrite: 0
Nitrate: 5
pH: 7.6

Symptoms and Treatment
How has your betta fish's appearance changed? Sapphire has larger fins: at first they were tattered, but last night I saw large holes in his dorsal fin and smaller holes in the anal and tail fins. Cobalt almost looks normal but I see dark spots in his pectoral fins, not holes, spots. He also had a pinhole in one fin but it healed with salt treatment.
How has your betta fish's behavior changed? Cobalt sometimes clamps his dorsal fin and hangs out at the top with his massive bubble nest. But he seems ok 90% of the time. Sapphire is scarier, he keeps lying on the bottom. And he no longer flares when he sees Cobalt. If he responds, it’s really minor.
When did you start noticing the symptoms? 1 week ago
Have you started treating your fish? I put them both in QT ½ gal jars inside the tank with salt (100% Water change daily. 1t/gal, then 1.5t/g, then 2t/gal.) When I saw that Sapphire was way worse I switched to Fungus Cure. 1st dose was last night.
Does your fish have any history of being ill? 9 months ago this started as a community tank with 10 tetra and 1 betta. The tetra were full of ich when given to us (a gift) and they got the whole tank sick. They survived the ich outbreak but were badly damaged and eventually died. Sapphire got fin rot dealing with the ich problem. He’s been fine since. They fought and hurt eachother badly a few months ago. Cobalt healed 100%, Sapphire never grew his full fins back.)
How old is your fish (approximately)? IDK. Sapphire is 2 inches long, Cobalt is slightly smaller.

I assumed the water got dirty somehow but tests were fine, better then expected. I did an 80% wc when it started. Then 4 days ago, a nearly 100% wc while I had them out for a QT water change.

Snails: We always had a few snails but not enough to be a problem. We went away for a few days in September. I have no idea what the fish-sitters did - but we had a horrible pest snail & algae outbreak after that. I got the algae under control. But the snails are getting worse. When we feed the bettas there’s never leftover food, so I assumed that the problem would correct itself. No food, no snail breeding, right? Right? But the snails keep getting more plentiful. Now they are everywhere. What are they eating?

[callistra]

That's generally indicative of ammonia burns but I see you tested and you have 0.. You do need to be doing weekly and not every other week water changes between 25-50% and that should keep the water quality a lot better and you should never need such large changes.. but I'd test anyway with the snails just to be safe.

Snails won't hurt them directly but they are very dirty and can muck up the water quality and that can lead to things like fin rot. I'd remove as many as you can see and keep at it until they're all gone. Snails will eat live plants and algae..

Sometimes poor diet can lead to thinner fins and problems like this. I don't know anything about your pellet so I did a search and found this: http://www.bettafish.com/showthread.php?t=123475 Maybe try another quality pellet.

Also they could be getting snagged on something? The panty hose test is the definitive test. Take any ornaments and plants and drag the hose across and if it snags them it will snag your boys.

The lethargic one may also be stressed by the other which has asserted dominance and no longer feels threatened. Make sure your divider is opaque and try to put lots of plants and things up against the divider so they can't see each other. If that doesn't help they may not be able to live next to each other. At this point I would put poor sapphire in a QT where he cannot see Cobalt and see if he does better away from the other.

Generally speaking, for minor fin rot 1 tsp per gallon aquarium salt preddisolved for up to 10 days is a good first stop treatment and will fix most issues. Stress coat at 1 ml per 1 gal can help them grow back faster once the rot is under control. If you are worried about it moving too quickly or other problems, like Sapphire's behavior, a good antibiotic would be KAnaplex or Maracyn Plus (not to be confused with Maracyn or Maracyn II). Why do you think Sapphire has a fungus? It sounds all bacterial to me.