A particular problem with rocks is the soluble minerals they may contain. They may contain metals or ores that over time will leach into the tank and prove fatal to plants and animals. There may be other pollutants on the surface of the rock that slowly dissolve in the water.
Granatic rocks (they have a speckled appearance) are the best, or quartz and other hard rocks that do not have leachable content. Rather than hunting for your own rocks, the best bet is to buy it at an aquarium store.
Gravel creates pockets of water poor in oxygen, supporting anaerobic bacteria potentially harmful to your guppies. Gravel also harbors pathogenic bacteria. Gravel traps food particles that spoil. Live food burrowing in gravel dies there, potentially fouling the tank. Although gravel also provides surfaces for nitrifying bacteria, and is used by undergravel filter systems as a colonization layer for beneficial bacteria, its potential for harboring disease outweighs its advantages. It’s final disadvantage for the guppy breeder with many tanks is that it adds to the cleaning chore.
If you want to use gravel, choose the color and granular size carefully. Guppies, like most fish, adjust their color to their surroundings. Dark gravel causes guppies to alter skin colors to darker shades. Light gravel washes out their colors. Small gravel or sand is better than large gravel. Food and other detritus falls into the crevices of large size gravel, decomposing and fouling the tank. Gravel is often plastic-coated because it can slowly, imperceptibly dissolve, changing the chemistry of the tank. Crushed coral, sea shells, dolomite and limestone release calcium and carbonates into the tank, raising its KH, GH and pH. Some people intentionally use these types of gravel to alter the chemistry of their tanks. Often bagged gravel is sold as “dolomite gravel.” However, if you are doing frequent water changes…or alternately doing infrequent water changes…the chemistry of your tank will be in constant flux. Guppies do not fare well under these conditions.
You can purchase gravel and sand cheaply at garden shops, big box hardware stores, local gravel and sand suppliers and grocery stores. However, most gravel is too large to be of practical use, and you do not know the chemical composition or source of the sand or gravel. The gravel or sand should be tested by pouring an acidic liquid (like vinegar) on it. If the gravel foams or bubble, you know that it is going to leach carbonates into your tank water. Or place the sand or gravel in an aerated tank and monitor its water parameters for a week. Another problem with gravel or sand purchased at stores other than fish stores is that it may contain detrimental bacteria or pathogens. Either boil the sand or gravel, or let it sit in water that has bleach added to it (a cup in a five gallon bucket).
Wherever you buy the gravel or sand, it should be washed thoroughly to remove the fine silt and dirt. Put it in a bucket of water and run water through it, stirring occasionally, until the water runs clear.
Other Decorations and Ornaments
What we have said about gravel applies to any object placed in the aquarium. You need to determine if the object is chemically inert. Most plastics, glass and ceramic are inert. Avoid black plastics. In general it is better to use white plastics that have not been dyed.
Such organic materials as driftwood and peat moss will leach tannins and other humic acids in the water. This will cause the water to soften and possibly it will lower the aquarium’s pH. Yellow water is not harmful, and the yellow color can be removed by adding activated charcoal to your filtering system.
Boil or bleach wood to remove pathogens and bacteria. This will help the wood to sink.