Is it normal for a tank to have a smelly odour, or as my other half says, "that tank stinks!" The tank is 720 litres,i change about a 1/3 of the water every 3/4 weeks. It does have heating cables with Dennerle deponit mix, and 2-3mm gravel on top. The depth of the substrate is around 10cm. Also T5's and CO2 etc. Any ideas?