Chicago Water Quality: Salinity, Nutrients and pH

Chicago tap water is fine drinking water and a poor starting point for a reef tank, and those two statements do not contradict each other. Municipal supply is treated to stay safe and stable across miles of distribution pipe, not to be biologically inert in a 90-gallon display. Disinfectant, dissolved minerals, treatment additives and whatever your building's plumbing contributes all arrive in the same glass. We have set up and serviced marine systems across Chicago and the suburbs for more than thirty years, and source water explains more stubborn algae and invertebrate problems than any other single factor.

What Chicago tap water carries before it reaches your tank

Public water has to hold a disinfectant residual all the way to the tap, and utilities across the region generally rely on chloramine, which is chlorine combined with ammonia, because it stays put far longer than free chlorine through a long distribution system. That stability is exactly the problem for an aquarium. Chloramine does not gas off if you leave a bucket standing overnight, and it is not neutralized by every product on the shelf. Whatever you dose has to break the bond and then deal with the ammonia left behind.

Everything else dissolved in the water gets summarized as total dissolved solids, and TDS is the number worth watching. Calcium and magnesium hardness, sodium, sulfate, silicate and nitrate all contribute to it, and none of them are touched by a dechlorinator. On a freshwater community tank that mineral load is usually a feature rather than a fault. In a reef, it is an uncontrolled variable stacked underneath a salt mix that was formulated on the assumption you started from pure water.

Treatment chemistry adds its own entries to the list. Many utilities dose orthophosphate or a similar corrosion inhibitor to keep metals from leaching out of older service lines, and phosphate is precisely what reef keepers spend time and money stripping back out. Your water provider publishes an annual quality report, and reading the one for your own address beats any general article, because source, treatment and plumbing all vary from one Chicagoland community to the next.

Chicago tap water shifts with the seasons

Source water is not a constant. Regional supplies move with rainfall, runoff, snowmelt and water temperature, and utilities adjust treatment through the year to match, so hardness, alkalinity and TDS sit inside a range rather than on a fixed figure. Anyone who measured their feed water once in October and never again is working from a snapshot rather than a picture. Two tanks in the same suburb, filled in March and in August, can start from noticeably different water.

The practical effect shows up in filter life long before it shows up in the display. When feed TDS climbs, DI resin exhausts faster and the membrane works harder, so cartridges that lasted six months start lasting four. A dual inline TDS meter, with one probe reading after the membrane and one after the resin, turns that into something visible on a Tuesday evening. Product water above zero means the resin is spent, and close to zero is not the same as zero for a reef.

Your own plumbing adds a second layer of variation. Water that has sat in a service line or an unused riser through a cold week picks up more from the pipe than water pulled from a busy main, and copper is the metal that matters most here, because invertebrates are far more sensitive to it than fish are. Flush the line before you fill a reservoir, keep water changes on stored purified water, and never fill anything from the hot side of the tap.

Phosphate, silicate and copper are what reach the display

Phosphate is the one most people meet first, usually as a green film they cannot scrub away. Reef keepers generally aim to hold phosphate somewhere around 0.03 to 0.10 ppm, and a top-off source carrying any measurable amount feeds algae every single day, quietly, without ever showing up in a feeding log. Nuisance algae is a nutrient problem before it is a lighting problem, and top-off water is the most overlooked nutrient input in the whole system.

Silicate is the second, and it announces itself as a brown dust across sand, glass and rockwork in the first months of a new tank. Diatoms need dissolved silica, and unfiltered or partly filtered water supplies it continuously. This is also the failure mode of tired DI resin, which tends to let silica through before it lets anything else through, so a tank that suddenly goes brown again is often telling you the resin is finished rather than that the tank is unstable.

Metals are the third and the most expensive. Copper leached from household plumbing does not have to be dramatic to matter; concentrations far too low to concern anyone drinking the water can still kill shrimp, snails and crabs, and corals do not enjoy it either. Chloramine brings a fourth problem in the form of ammonia, which a mature filter handles quickly but a newly cycling tank does not. None of these are visible, and all of them are removable.

Why we run RO/DI on every marine system we service

Reverse osmosis with deionization is the answer we install because it addresses the whole list at once. Sediment filtration protects what follows, carbon strips chlorine and chloramine, the membrane rejects the large majority of dissolved solids, and the DI resin polishes what is left down to zero TDS. Carbon capacity is the stage people underrate: chloramine needs real contact time, and a chloramine-rated block or a pair of carbon stages protects the membrane, which chloramine will otherwise degrade.

Maintenance is what separates a working unit from an expensive ornament. This is the review we run when a tank keeps fighting algae despite good husbandry:

• Confirm product water reads zero TDS, and replace DI resin as soon as it does not

• Change sediment and carbon on schedule, since carbon is what keeps chloramine off the membrane

• Check incoming line pressure, because a low-pressure feed lowers rejection and wastes resin

• Flush the membrane after long idle periods and keep the storage container covered and food-safe

• Mix and circulate saltwater fully before use, then check salinity against a calibrated refractometer

Buying purified water instead of owning a unit is a reasonable choice for a small tank, though it stops being convenient somewhere around the point where a water change means five heavy trips. Either way, test what you bring home with your own meter rather than trusting the label on the jug. Our guide to RO/DI water and mixing salt properly covers the mixing side of this in more detail than we can here.

Treated tap water suits freshwater tanks better than reefs

Freshwater keeping is where regional water becomes an advantage rather than an obstacle. Hard, alkaline water is what Lake Tanganyika and Lake Malawi cichlids evolved in, and livebearers such as platies and swordtails do well in it too. A conditioner rated for chloramine, dosed for the full volume being added, handles the disinfectant and the ammonia together. Soft-water species like discus, chili rasboras and most Apistogramma are the ones that want purified water blended back down.

Testing is what keeps that decision honest, and it takes ten minutes. Check pH, KH and GH on the tap water itself, not just on the tank, so you know what a water change is actually adding, and repeat it a couple of times a year rather than once at setup. Our walkthrough of a freshwater testing routine that catches problems early lays out which parameters matter and how often to look at them.

The mixed approach is what we set up in most homes that run both kinds of tank. Freshwater displays get conditioned tap water, marine systems get RO/DI, and the two never cross. Topping off a reef with tap water even occasionally is worse than it sounds, because evaporation removes only water and leaves every mineral behind, so anything you add through top-off concentrates in the display rather than passing through it.

Key takeaways

• Chicago tap water is treated for safe distribution, which means chloramine, dissolved minerals and sometimes phosphate-based corrosion control.

• Chloramine does not evaporate out of a standing bucket, and it leaves ammonia behind when neutralized.

• Hardness and TDS shift across the year, so feed water is worth retesting seasonally rather than once at setup.

• Phosphate feeds algae, silicate feeds diatoms and trace copper harms shrimp and snails long before it bothers fish.

• RO/DI is the fix for marine systems; conditioned tap water remains perfectly good for most freshwater tanks.

Frequently asked questions

Can I use tap water for reef tank water changes?

We do not recommend it. Even well-treated municipal water carries minerals, nutrients and trace metals that a salt mix never accounted for, and water changes are supposed to reset a system rather than reload it. If you are stuck mid-emergency, conditioned tap water beats a tank that has crashed, but treat it as a one-time rescue rather than a routine.

Does letting water sit overnight remove chloramine?

No. That advice dates from when free chlorine was the common residual disinfectant, and chlorine does dissipate with time and aeration. Chloramine is deliberately more stable and stays in the water essentially unchanged. Use a conditioner that specifically states it neutralizes chloramine and binds ammonia, or remove it with sufficient carbon contact time in an RO/DI unit.

How do I know when my RO/DI cartridges are finished?

Watch product TDS rather than the calendar. DI resin is done the moment output stops reading zero, which often shows up first as returning diatoms. Sediment and carbon are replaced on time regardless, because carbon protects the membrane from chloramine, and a membrane damaged that way keeps producing water while quietly rejecting far less.

Work with Marine Concepts Inc

Water problems are usually source problems, so tell us what your tank is doing and we will start by testing what goes into it. You can see what our maintenance visits include for homes and offices across Chicago and the suburbs, and RO/DI cartridges, test kits, refractometers and salt are available for same-day pickup at Marine Concept Supply in Wheeling, IL.