my plecos died unknowingly

Question
hi karen,
my is John, you probably remember me asking question from u weeks ago, now i have more serious problems. i have a 55 gallon w/ 1 PLECO,, 1 knife fish, 1 bala shark,1 catfish,1 dwarf gourami,3 small sunset gouramis, and 4 black tetras. my Also, i have a 10 gallon w/ 1 PLECO, and 4tiger barbs.
First my PLECO in my 10 gallon tank began showing symptoms of "weakining" there's no ich or fungus but i've noticed that my white rocks were turning "green" probably algae then i checked my water real quick-(nitrates 20ppm,nitrite-0,alkalinity average,water-hard,and 6.4 PH). However, my pleco died which was 2 days ago after checking the water. Now, my pleco in my 55 gallon showed the same thing, he's weak when i saw him last night, i've had him for more than 6months now. In addition, i've noticed that my pebbles in my 55 gallon were "brownish" and the plastic plants were brownish too, probably algae again. Then, i checked my water-(Nitrates 20ppm,Nitrite 0,Alkalinity-Moderate,Water is Very Hard, and PH is 6.2). Unfortunately my 2nd pleco died unknowingly. what could have killed them? i do water changes every 1 or 2 weeks or so. i feed my sinking tablets for my bottom feeders like my pleco and catfish alike. was it the green thing in my 10 gallon, or the brown thing in my 55 gallon. my 55 gallon is 6-7 months old already. i feed my fish bloodworms often, flakes sometimes, and even brine shrimp. i couldn't help thinking that i failed my plecos, now both of them are dead. what happened?  

Answer
Hi John,
These Weakening signs you mentioned are definitely without a doubt a clue to why your poor plecos died. By weakening do you mean that they stopped becoming as active? Or did they start to appear thin and underweight? Many times, even if algae is present, plecos can starve because the type of algae isn't the kind they prefer most. Sometimes plecos may not get enough nutrition from sinking foods either. The best way to tell is to have checked closely on your plecos' condition. Did they appear very thin? For the most part, well-fed plecos should be full bodied and "bright eyed" so to speak.

I'm certain the green and brown stuff in your 10 and 55gal are algae. Green isn't too much of a concern as long as it doesn't take over but the brown algae may be a sign that there is too many nutrients or pollutants in your water. It could mean that the gravel needs more frequent vacumming and more frequent water changes as well as filter cleanings.  

What kind of food did you feed for your plecos? Did you supplement their diet with Zuchinni and romain lettuce? Did you drop their food in when all lights were out? I don't mean to blame you are anything, but it could have been possible the two plecos weren't getting enough food for themselves--maybe the other fish were ruling the food and the plecos got very little to none. It's very hard to know. But it is unusual for yours to both behave the same way before passing on.

But if the other fish are vibrant and healthy then I would suspect that the two poor plecos weren't getting enough to eat.

I really hope this helps. I'm very sorry to hear about your poor plecos... Feel free to write if you have anymore concerns.

Best wishes as always,
Karen~