Q. You recently made some suggestions for avoiding green water problems. Can you give me some tips on how to get rid of it once it arrives? I've tried water changes, and the tank will look good for a day or two, but then it's right back to pea soup!

A. The reason water changes seem to work temporarily, but not for any length of time, is that these types of algae are very prolific and do well in conditions quite similar to those needed for good growth of higher plants. If even a few algae cells are left in the tank, they can quickly multiply into huge quantities.

The good news is that green water blooms are not very often "dangerous" as long as the water does not turn so opaque that it blocks light to the higher plants. Green water is usually no more than an unsightly annoyance, and even if you do nothing but continue routine maintenance on the tank, it usually goes away on its own eventually.

If you just can't stand looking at it any longer, though, I suggest the following approach. Do several large water changes, both to remove any excess nutrients and to reduce the number of algae cells. At the same time, gravel vacuum any unplanted areas of the substrate thoroughly. Then, really darken the tank by turning out the lights and wrapping the tank in towels to avoid ambient room light.

Do not feed the fish during this period, but you may take a peek now and then to make sure the fish are doing okay. Depending on the fish load, it may be necessary to install an airstone to increase water movement, because the plants will not be producing oxygen with the lights out.

In about five days, unwrap the tank. You should find that the water is crystal clear. As long as the underlying nutrient problem that caused the bloom was resolved first, the green water should not return.

Laterite
Q. I am busy replanting my 20-gallon freshwater tank. However, I have gravel, and only gravel, in the bottom of my tank. You mention using laterite. I am truly a novice — I do not know what laterite is and did not find it in the dictionary. Could you please let me know what it is? This would be greatly appreciated.

A. Laterite is an iron-rich clay from tropical, or once tropical, areas of the world. It is common in South America and Asia and can be found in lesser amounts in other areas as well. Not all red clays are laterite and not all laterite is suitable for aquarium use. Many sources contain high levels of bauxite (a source of aluminum) and can be toxic under aquarium conditions. Some sources of laterite are very fine in texture and have a tendency to stay suspended in the water for long periods of time. The best laterite for aquarium use is high in iron, low in aluminum and of a coarse enough texture that it will, if disturbed, settle back to the bottom within a reasonable length of time.

There are many other substances that can be used to enrich a plain gravel substrate in order to obtain good plant growth. Some people use local red clays and soils or red art clay. Results with these substances are luck-of-the-draw. There are too many variables to guess which ones will be suitable.

If you want to experiment, do so with caution. But in my opinion, no other substances combine the benefits of ease of use, safety and good plant growth as well as laterite does. It can be interesting and beneficial to experiment with other substrate additives as you become more proficient at maintaining healthy, stable planted tanks, but please do yourself a big favor and set up your first planted tank with a commercial laterite supplement.

Finding Laterite
Q. Since I returned to the hobby a few years ago I have become more and more interested in growing aquatic plants in my aquariums. In the books and articles I have read on this subject one word pops up again and again: laterite. It sounds like a very good additive, but I have never seen any book or article that tells me where to get it. I have never seen an ad for it and I have never found a pet store that carries it. In fact, I've only found one pet store proprietor who had even heard of laterite, and he had no idea where to get it. Can you offer some sources? Thank you.

A. Because so many people have requested sources for laterite, I am bending my self-imposed rule not to "promote" specific brands by listing a couple here. I'm sure there are other adequate products and there are certainly other distributors. But the following are sources I have personally used and feel confident recommending.

Dupla laterite (and other Dupla products) can be purchased through many local pet stores. Please, whenever possible, remind your local retailer of your interest in planted aquariums and urge them to support your segment of the hobby. Then do your part by supporting them!

If your store doesn't stock Dupla products, ask if they can order them for you. They can order them from J.P. Burleson, the U.S. distributor for Dupla products. If your local store can't or won't order it for you, two good mail-order sources of Dupla products are Monolith Marine Monsters (617-494-8396) or Pet Warehouse (800-443-1160). Both of these companies offer a wide selection of supplies and equipment for the planted aquarium. Another good American source of laterite is Schoeller Enterprises (612-432-9608).

Weighted Plants
Q. I have a 35-gallon tank with catfish and loaches, and pea-size gravel. I put some live plants in plant-plugs in little plastic pots, but the fish keep knocking the plants over. Have you seen any small plant pots that are weighted to stay upright?

A. Unless you have unusually large catfish and loaches in your 35-gallon tank, once your plants get well rooted in the gravel itself, the fish shouldn't be able to uproot them. The root system of a healthy plant is much larger than those little pots! I'd take them out of the pots and let them do their thing in the substrate itself.

Your gravel is larger than optimal, and I suspect that you don't use any substrate amendments either. Laterite balls inserted at the base of the plants will encourage them to spread their roots out. You can keep the fish from disturbing the plants until they get rooted by placing stones around their bases that are too large for the fish to move.

If you find that, in spite of this strategy, your fish are still too rough on your plants, the best bet is to pot them up in regular clay flowerpots. These will give your plants more room while still protecting their roots. They are also heavy enough that most community-type fish can't dig them up if they are buried in the gravel. If your gravel isn't quite deep enough to hide the rims completely, they can easily be camouflaged with stones, driftwood or Java moss (Vesicularia dubyana).