mystery snails and thin white worms

Question
I have a thirty gallon tank that I have maintained happily for the past seven years.  In the past three months I have had six fish die (five species, including tetras, plecostamus, upside catfish, 2 dalmation mollies, and a platy)!  Whenever I check the water chemistry it appears to be very good. I do a 20% water change once a month. When my plecostamus died I began to notice an increase in algae on the tank walls. After watching this for a month I thought maybe the plecostamus was keeping that in check (he was the oldest fish...7 years.  I also purchased two mystery ghost snails (black and white) thinking they could also help the algae issue.  

Now I am worried that I shouldn't have bought the mystery snails...that there will be a population boom and my tank will be overrun with snails!  
1# Is there a fish that will keep the snail population in check that I can use with my current inhabitants?

I was cleaning my gravel while doing my water change and saw the filamentous-like worms swimming.  Uugghh.  I am not a parasite person...please tell me these are free-living.  I've read that doing the daily water changes for a week or so and feeding less will rid them.

2)Could the presence of these worms indicate something that would kill my other fish (the last platy that died appeared to have dropsy, where it kind of swelled up like an orange pineapple or puffer fish...very uncomfortable)?

Answer
Hi Gina,

Mystery snails are not prolific breeders in aquariums.  You're in no danger and they are an ASSET to the tank.  I know other so-called experts hate them, but I find them quite tranquil.  I keep several in my planted tanks.

They don't eat the plants, either...just the algae.

I'd like to know how many fish you have in the 30 gallon.

What sizes?

What breeds?

I have a feeling this issue has been brewing a long time.

Algae is the first sign of filthy water.  Your water quality is bad.

20% isn't enough if the filter isn't sufficient and there are more than a couple fish in a tank.

You lost six.  This leads me to wonder how many are in there.

Also, your pleco, and any goldfish could really be contributing to this problem.  Plecos eat algae, but they also poop and filthy the water.

Clean it.  Change filters.  Pads.  Water every month.  

Cut down on the amount of fish kept.  Feedings should only be once a day.

I look forward to your answers to help you. :)

Happy fish-keeping.