Characteristics of San Francisco Tap Water
by Steve Dixon

San Francisco is blessed with very pure high quality water from the Hetch-Hetchy and surrounding catchment areas in the Sierras. Our water is quite soft (1 - 3 degrees of GH and KH, which varies seasonally with the snowmelt) and contains very low levels of nutrients required for plant growth. Simply put, San Francisco water will not support vigorous aquatic plant growth without supplementation of key nutrients. In my view this is the principal reason San Franciscans often end up learning how the grow plants "the hard way." Other folks (Marin County comes to mind) have an easier go of it because their water contains higher levels of essential nutrients (and thus tolerates more user error). The beauty of San Francisco’s pure water is that it forces us to learn a little bit about plant nutrition.

Macronutrient Levels[1]
Range (ppm)
Average (ppm)
Nitrate
<0.4
<0.4
Phosphate
<0.05
<0.05
Potassium
<0.2 - 0.9
0.6
Sulfate
0.8 - 14
7.4
Calcium
<3 - 17
9
Magnesium
0.2 - 7.3
3.8
Carbon (C02)
1 - 3[2]
1 - 3
Alkalinity (as CaCO3)
15 - 66
41
Hardness (as CaCO3)
14 - 66
40

Micronutrient Levels
Range (ppb)
Average (ppb)
Iron
<5 - 46
24
Copper
<1 - 3.4
2
Manganese
<3 - 3.4
2.5
Zinc
<5
<5

Target Nutrient Levels (water column)

Macronutrient Levels
Range (ppm)
Multiple of Tap Water Average
Nitrate
3 - 5
7.5x
Phosphate
0.5 (pulsed)
10x
Potassium
15 - 25
25x
Sulfate
no info
no info
Calcium
~18[3]
2x
Magnesium
~8[3]
2x
Carbon (C02)
15 - 25
10x
Alkalinity (as CaCO3)
15 - 66
2x
Hardness (as CaCO3)
14 - 66
2x

Micronutrient Levels Range (ppb) Multiple of Tap Water Average(ppb)
Iron[5]0.104x

Rules of Thumb for Achieving Target Nutrient Levels

The following very rough rules of thumb assume adequate light (say, 2 watts per gallon or higher), CO2 supplementation (say, 15 ppm) and regular water changes (say, 25% every 2 weeks). At lower light or CO2 levels my guess is that these rules of thumb should be reduced significantly, perhaps greatly. Heavy feeding and higher fish loads would seem to suggest lower nutrient supplementation; however, I have found significant macronutrient shortages in higher fish load tanks, including several with fairly heavy feeding regimens.

Macronutrients

Potassium and Nitrate; Supplement with KNO3 (Potassium nitrate, which is available from Dave Gomberg). 1/8th to 1/4th teaspoon per 50 gallons (total tank size) per week. Tanks with very high growth rates may benefit from up to twice this amount, but it is quite important NOT to let NO3 levels build up much over 10 ppm. Several of our favorite species (Rotala macrandra and Eusteralis stellata, for example) do not do well at higher NO3 levels.

Phosphate; Supplementation may not be necessary; however I have found that my tanks benefit from "pulsing" very small amounts of phosphate once or twice per week. The plants usually take the phosphate up in a few hours so that the water column appears to be phosphate free; however, the benefit to my plants is quite apparent. One-quarter of a 1/10th gram teaspoon per 50 gallons (total tank size) once or twice per week.

General Hardness (GH): Several of us use Seachem’s Equilibrium with good results, and quite interestingly, we have stumbled into the same dosing level. 1/4th teaspoon per 5 gallons of water changed. This will maintain about 5 degrees GH during most of the year and about 3 degrees GH during the spring runoff. Equilibrium contains quite a bit of Potassium which may be the reason so many of us notice a benefit from using Equilibrium.

Carbonate Hardness (KH or total alkalinity in our case): I use between 1/8th and 1/4th teaspoon of Seachem’s Alkaline Buffer per 5 gallons of water changed. I believe regular Baking Soda will work as well. Calcium carbonate (CaCO3) will increase both KH and GH. Epsom salts can be used to bring up the Magnesium portion of GH (if one is using Calcium carbonate to increase both KH and GH). I play around with 1/4th teaspoon amounts and use the Tetra KH/GH kit to check the levels once in a while.

Micronutrients

Iron (as a proxy for all micronutrients)[6]: 15 - 20 ml/Tropica Master Grow per 50 gallons (total tank size) per week. Depending on the growth rate more than this amount may benefit the plants. After a few months of use you will be able to judge the level by looking for rich green new growth in your plants. If the plants are coming in rather pale and only greening up after a month or two, then you should increase the level of micronutrients. This was Claus Christensen’s principal observation on his visit to the West Coast earlier this year.

Bay Area aquarists have also found the Seachem products to be quite good. I have not worked out the dosing levels yet, but Seachem Fluorish and Seachem Fluorish Iron used in the recommended amounts should result in excellent plant growth.

A Final Word of Caution

When you first discover that you can almost watch plants grow when you provide them with the nutrients they need, the temptation to overdo nutrient supplementation is virtually irresistible. I don’t know of a single person (myself included) who didn’t succumb to overdosing nutrients when they made this discovery. It is just too much fun when you see plants thriving and growing like mad. Perhaps more will be better! Wrong! Just enough for the growth characteristics you want is better.

Perhaps a personal example will serve to illustrate the point. Over the past week or so in getting ready for my open house on the 9th, I wanted to trim my Glossostigma elatinoides lawn because it simply had grown into a mountain. I trimmed it back severely, but wanted it to grow back quickly (in about 10 days). To accomplish this I increased the nitrate to about 10 ppm and raised the iron level to 0.15 ppm and pulsed phosphate every second or third day. The result: Rotala macrandra clearly shows signs of getting a bit too much nitrate; the Eusteralis stellata growth rate actually slowed (and may form rosettes at the growing tips), and the glosso hasn’t grown back either!

Be careful out there. It’s a jungle (if you overdose nutrients)!

    Footnotes
  1. From San Francisco’s April, 1998 Water Quality Report No. 5
  2. For years I have commented on the APD that the "equilibrium" level of CO2 in the water column of aquaria is between 2 and 3 ppm. Using the chart, I have always observed this level of CO2 in my tanks if I turn off the CO2 system. A couple of months ago George Booth pointed out correctly that the equilibrium level of CO2 and water is under 1 ppm. The 2 – 3 ppm that I observe in my tanks includes a small amount of CO2 being generated by the bioload in the aquarium (fish waste, decaying plants, peat granules and the like).
  3. I don’t have a clear sense of how much Calcium and Magnesium we need for good plant growth, but to keep the GH around 3 degrees we need to double (approximately) the levels found in our tap water. The preferred ratio of Calcium to Magnesium is said to be between 2 : 1 and 4 : 1. We are quite fortunate to find our tap water providing these nutrients in this ratio.
  4. See previous note.
  5. We use Iron as a proxy for all micronutrients since as hobbyists, we can’t measure numerous micronutrients. If we use a “balanced” micronutrient mix and maintain Iron levels at 0.1 ppm or so, we assume we have a sufficient supply of all micronutrients.
  6. We use Iron as a proxy for all micronutrients since as hobbyists, we can’t measure numerous micronutrients. If we use a “balanced” micronutrient mix and maintain Iron levels at 0.1 ppm or so, we assume we have a sufficient supply of all micronutrients.

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