Freshwater Aquarium Water Quality Issues

Question
Hi. I just did a wholesale cleaning of my tank 3 weeks ago (water, sand, plants, fish all out to scrub the thing). I did this primarily because my tank was collecting weird "rust" looking spots on the glass. The "rust" looking spots are back and I can't figure out what the issue is (books are useless). Can you advise what it is, what causes it and how I can fix it?

My PH is high at 7.6 (I can't seem to get it to budge off of that number... ever!)
My nitrites are at .25ppm.
My ammonia is at 0ppm.

The tank contains an 8" Leopard Plecosathmus, a rope fish, a fire eel, a spotted catfish, 3 Gouramis, 2 rainbows (?), and a funky little black shark. The tank is 35 gallons.

Any advice you can provide is appreciated. Thanks in advance.

Answer
Hi Deirdre;

The brown stuff is just algae. No need to do much except wipe it off. Brown algae doesn't require much, so it occurs in tanks with areas that have low light, not-so-good water quality or in tanks that have recently gone through a break-in period. Usually plecostomus eat it. Yours may be feeding on green algae enough that he doesn't want to eat the less flavorful brown algae. Or, you have been overfeeding, which we all do from time to time. Plecostomus that are too well fed with supplements of excess food will refuse algae of all kinds.

You will have to do some water changes to get the nitrites down. The beneficial bacteria is trying to re-establish itself so it can consume it again. Major cleaning kills some or all of this bacteria that your tank needs. Sort of like starting a new tank. Remove 25% of the water and replace with fresh water every day until the nitrites go down to "zero" and stay down. Nitrite is highly toxic to the fish and they can't absorb oxygen very well when it is present. Feed very little food to the fish right now. The more you feed, the higher the nitrites will go because the fish put out more waste. They may not be as hungry as usual while nitrites are high anyway. Once it finishes 'cycling' you can continue with normal feeding amounts.

Followups welcome

At Your Service;
Chris Robbins