I have been pleased with how my tank looks since I set it up a few months ago. The water was clear and not much algae. But last week I noticed a black algae on the plastic plants and starting to spread to the gravel. It was easy to take out the plants and clean them. I did not know the best way to clean the gravel. Rather than remove the gravel, I decided to put my fish in a bucket with water from the tank. Then I drained 90% of the water. Then I stirred the gravel up and agitated it a lot to hopefully clean it up, then I suctioned out as much as I could of the remaining filthy water. I refilled the tank and added some stress relief that is suppose to remove any chlorine etc. I put it all back together and put the fish back along with the old water. I thought the 5 gallons of old water and the bio-wheel would provide enough beneficial bacteria. So it's all back together and looking clean again. However, the fish haven't been as active. I checked all the parameters and they are good. But they are not as excited to eat like they were in the past. They used to swim up to the top and wag their tails and would eat the food before it sunk to the bottom. Now they don't do that. They don't appear to be stressed. the breathing is normal. The are timid and shy. What is going on?