I had two plecos (not sure what type) in with my betta and african dwarf frog. I fed the betta and frog bloodworm and the pleco algae wafers. But a few days after i got them one died. Then a week later the other one died. I checked the water and temp after the deaths but it was completely fine. Why did they die? Was it because they ate some of the bloodworm? Can they eat bloodworm?
it is possible they were both sick before you got them, especially if they both came from the same vendor / tank.
otherwise, make sure you checked for ammonia/nitrites/nitrates/PH/KH/GH and all was within the normal perameters for that fish, here are some links I have found as I just got a dwarf pleco several days ago myself. I love the little guy / girl, not sure and didn't ask.
http

/www.aquariumlife.net/profiles/plecostamus/bristle-nose-pleco/100049.asp
http

/www.aquaticcommunity.com/pleco/care.php
hope these sites will help. Insofar as specific cause of death, hopefully someone more experienced than myself can put together the pieces of that puzzle. It is not an easy thing to determine without more knowledge, especially if your other fish are fine.
also found this:
Otocinclus sp. Oto / Dwarf suckermouth catfish / Dwarf Pleco
Adult size 1.5" (4cm)
Good plant cleaner: Green Surface, Brown/Diatomic; Won't damage plants
Often considered the best algae eater for small tanks. Well suited to community tanks but really should be kept in groups of 4 or more. They are very sensitive to water parameters and
have a habit of dying for no apparent reason, however once established they do a fine job of keeping the tank algae free. The typical rule of thumb for picking out Otos is ask how long they've been in the shop and look for fat bellies. An Oto that's been in the LFS for over 3 weeks and has a round tummy will probably make it in a well-kept aquarium.
Resized to 94% (was 640 x 480) - Click image to enlarge
picture by Nospherith
All of the following plecos require supplemental feeding to survive. They need lots of food and make lots of poop. We're talking foot-long strands of fish poo all across the aquarium here; if its not pooping a lot, your pleco isn't healthy. Count yourself as warned.
Sorry i could not be more help.