In my 5 gallon tank I have 1 mail veil tail betta and 4 golden pristella tetra's. I have never had issues with this tank in 4 mo's. Now just about a mth ago I noticed a greenish/brown algae growing on the blue/white stone and some on the walls of the acrylic tank. I do monthly water changes-about 25% each time. Also use water softner. A wk ago I removed 25% of water and (as directed) removed the carbon filter for 6 hrs after adding Tetra algae control. I then added new water back with water softner. Here I am a wk later and there is algae forming on the stone again and in a couple of spots on the inside of tank. I usually have a very clean looking tank. Suggestions?