pleco died mysteriously

Question
hi Chloe,
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,
The brown algae you are describing in your tank is usually the cause of a newly setup tank. This usually goes on its own with a little help of daily water changes of 30%. Its either that or you are overfeeding your fish. This could have caused the deaths of your Pleco's. Your pleco would greatly benifit from daily water changes.
Good luck!

Chloe.