I'm not an expert on plecos, but can offer the following.
They get BIG and a 30 gal is a little small. You might want to look into smaller species.
A diet high in vegetable matter is essential, algae is just a hobby with these guys. Fresh Romaine lettuce, zucchini, boiled Spinach, algae wafers, bottom pellets for plecos, etc. They will consume large amounts. A diet containing fish or animal fat can cause them problems.
I think you might want to revisit the diet issue, and check with your supplier to see is their diet was adequate before you bought them. (If he will tell you
)
Regards,
Jay
pleco problems 
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I'll say it for them:
