Basics : Rocks
Whether you decide to buy or just collect them might depend on how much you need.
Rockwork has a variety of functions, so some selection criteria should be considered. Examples are as shelter (spawning sites, territorial boundaries or for hiding fry), appearance (beauty or just hiding heaters and hoses), structural (tiering gravel or holding wood down), pH altering (dependant on mineral, though the effect is almost exclusively upwards), water-hardening (dependant on mineral) and biological (providing a surface for bacteria, algae and microscopic organisms) or to attach certain types of plants (Java ferns, moss, Riccia, Anubius etc).
Not all rocks are aquarium safe, but there are methods to test them. Copper is extremely toxic, it's effects start with parasites, then as the dosage increases, invertebrates like snails and then to fish. Not all rocks are suitable for aquariums (due to their size, shape or effect on the water), and installation methods will vary. Some rocks are completely inert (will not react with water like slate or granite), while others are very reactive (Utah Ice melts in aquarium water).
The greater the porosity of the rock, the more potential for hazard (or benefit) it will have. This includes pathogens (from stones from natural freshwater environments), to anything which might leech out (typically natural calcium, but may include toxins). Concrete products will temporarily leech significantly.
When you get undesirable algae growth on your rocks, turn them around. Smooth rocks are easier to keep clean than porous rocks, though it's a cosmetic issue rather than a hazard.
Details...
BUY or COLLECT
The rocks found in your local pet shop will generally be completely safe, but will include both inert and non-inert types. The retail cost is quite high, mostly reflecting the cost of transport, and not the actual materials. If you are stocking a small aquarium, or are just looking for a few unique pieces not found in your area, or natural stones are not available where you live, (or they are under a foot of snow), then consider pet shop rocks. Although they have the highest prices, they may also have some of the more interesting types to tempt you (ie: petrified wood, zebra rock, rainbow slate etc). For any significant amount of rocks, visit your local quarry and home renovation stores to compare price & availability. Rocks collected from natural environments can also be used. Contact your local aquarium society to see when they are planning their next trip, as they usually know the best places to go.
SELECTION
Select according to application. Covering the bottom of a tank with smooth river stones might be very attractive and ideal for hiding fry, but gravel vacuuming would be nearly impossible. With tall fish (Discus, Angelfish), thin slate standing on end might be used as a spawning site. With African mbuna, stacking coral or tufa chunks provides both shelter and helps water parameters. For soft water tanks which are too low in buffer, a few pieces of a calcium leeching stone might help (aragonite, dolomite, limestone, marble, coral, tufa etc), either in the filter or directly in the tank. These stones are almost always beneficial for hard-water tanks. The amount the rock reacts will be a function of the difference between the water and the rock's surface area (and how it is bonded to the stone). Generally, the effect is gradual and much more fish-friendly than using chemicals. Contrasting your substrate (zebrarock on white or black gravel, rainbow slate on blue gravel, blue glass on red gravel), is a common technique, or go with more natural tones (beige, black, brown, grays etc). Changing colours as you move across the tank is a pleasing effect (if not overused). This works well with foreground rocks blending into taller background rocks of a different shade. Many neutral inert stones come in large chunks (granite, picture stone, travertine). Sedimentary rocks such as sandstone have attractive layers which can be used to good effect.
TESTING & PREPARATION
A simple test it to drip a strong acid on a rock and observe the reaction. If it fizzes, then it will leech calcium into your water, making it more alkaline and harder. Vinegar is a bit too weak an acid. The pH-down product is a satisfactory tester. I think the acid reacts with the carbonates, which generally indicates a high degree of calcium. The test is not completely accurate. Minerals low in carbonates may still leech calcium (ie: Utah Ice), and some minerals may cause the pH to drop in the right conditions.
Avoid green in rocks as this is the color of copper in a normal oxidized state, and copper is toxic. One test method is to put a representative sample of the rocks collected in a pail of water with some fish (ie: white cloud minnows). If after a week or so, the minnows are fine, they you have some degree of confidence that whatever the rocks might have was not toxic in the high concentration which would have built up in the pail of water. Perform pH, gH and kH measurements as the week progresses (minimum 3 test sets). Pour your source water into the pail and a glass of water (your control group). Measure the pH, gH and kH before adding the rocks (sample #1). Repeat your measurement using the water in your glass after 24 hours (sample #2). This is where your tank water would stabilize, if there was no influences (allows any outgassing to have completed and the pH would have settled). After a week or so, measure the pH, gH and kH in the pail (sample #3).
The difference between sample #1 and #2 is mostly indicative of how much dissolved gases are in your water. This will be covered in more detail in a Basics water section (when I write it). Your current interest is in the difference between sample #2 and sample #3, which indicates the trend the rocks will take you in, but not the finished value. It is possible to leave the stones in longer to better estimate the finished value, but this is of questionable value, due to the parameters of your source water and your rate of water changes. The question you are currently answering is if the rock leeches, and how reactive does it appear to be (which requires more evenly spaced measurements).
Like driftwood, the rate at which rocks leech tends to diminish (depending on the type, the calcium now deeper inside takes longer to escape to the water, and bacteria begins coating all the surfaces). Some rocks will literally melt or progressively disintegrate in your water. The rate of disintegration is a function of the difference, so very acidic water will cause a stronger reaction than very alkaline water. This makes rocks ideal as a self-regulating pH adjuster, while usually also adding carbonates which build up your kH (stabilizes your pH).
INSTALLATION
If you have fish which dig, (ie: most cichlids, loaches, corys to an extent etc), rocks should not be placed on the substrate. Any excavation can cause a rock to shift, trapping the fish underneath. The worse case scenario occurs when the rock tips forward with enough momentum to crack your glass side.
Ideally, gravel should be put into the aquarium after the rocks are all in position, so the rock structure is solid and not prone to any influence from digging. Placing rocks directly on the glass bottom will usually not cause any problems, but it does cause very singular points of pressure (typically 3 points per rock). If the result is a few scratches on the glass, this is of little consequence. If you will be piling a significant amount of rockwork up, then it's better to place a sheet of thin styrofoam on the bottom of the tank. Look for a 1/2" or 1" thick extruded polystyrene (blue from DOW chemical, pink from Owen Corning, readily available at a home renovation supplier). The order of installation becomes 1) styrofoam, 2) rocks, 3) gravel and lastly 4) water. The buoyancy of Styrofoam makes installation while there is water in the tank a very difficult exercise (about 60 lbs of buoyancy per cubic foot). Expanded polystyrene can also be used (usually white, constructed of little bubbles), but this will damage and break down more easily, so extra care is required.
CONCRETE & CONCRETE PRODUCTS
Like many natural calcium leeching rocks, concrete will also harden water and raise your pH, however, unlike natural rocks, it is an uncontrolled blast, rather than a progressive effect, so it's very undesirable, except in the largest bodies of water which can absorb their effect (ponds). If you want to use concrete products, or have made a structure out of concrete, then you need to place the product in a leeching tank (with no fish). Changing the water regularly will progressively dilute its effect. Adding acid will accelerate the leeching. Once it has finished leeching, it is usually safe for aquarium use (you can use the fish test outlined above), though it will still have a lingering effect for some time.
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