A novel way of getting rid of watermarks in your aquarium.


Having arrived early for a meeting in Makati, I found myself killing time in a mall. With nothing to do, just like any other fishkeeper, I entered a hardware store. I guess you’d have to be a fishkeeper to relate; for me, next to a fish shop, a hardware store is an amusing place to be to look for hardware you can use for your aquarium. Naturally I rummaged through lighting systems, plastic tubs, electrical devices, and all other stuff. Each time, I asked myself, “What can do with this?” But cheapskate that I am, I ended up purchasing a piece of sandpaper which cost me Php24.

Water Mark Irritations

One of the most irritating things that we encounter in the hobby is water marks on the tank panels. For many years, I have been searching for a good way to remove these—and I may have just found a solution that is quite easy to use.

Water marks are created when water from our tank dries up. Naturally, water in our tanks is not pure H2O but a mixture that contains other elements. When water evaporates, some minerals like calcium are left on the glass panel.

This leaves an unsightly white mark on your glass which makes your tank look dirty and old. This mark is very stubborn and hard to remove. I have tried many ways: using acids like vinegar and muriatic acid to neutralize its alkalinity; using a cutter blade to shave off the calcium deposits (which sometime scratches the glass); and even using a sponge to vigorously buff the glass, which has hardly any effect.

Where–and How–They From

Water marks are something all fishkeepers have in their tanks, and there are four ways they are created.

1. Water marks at the waterline.

The surface of our water is where evaporation takes place; therefore, it is here that dried-up calcium deposits gather. This is unavoidable because all tanks will develop white horizontal marks. These white marks develop across the panel from one end to the other in all 4 panels of the tank. They oftentimes develop multiple layers, depending on how much evaporation took place.

So long as evaporation takes place (and this is almost a 100% certainty unless, of course, you have a frozen tank, in which case it is a 100% uncertainty), your tank will develop these horizontal water marks. A way to conceal this is to raise your water level to the level that the plastic strip covers the waterline, and make sure your water level stays there. This way, all the water marks that develop are behind the plastic strip and will not be visible from the viewing standpoint. However, this is very hard to achieve; we cannot always be vigilant with a concern as trivial as the water level. So face it: chances are, your tank will develop these horizontal white marks whether you like it or not.

2. Water marks in the area where the return flow of the filter is located.

Whatever kind of filter you use—whether overhead, sump, trickle, hang-on, and even sponge and undergravel—it will have to return water to the tank. All of these will create a splash when water is returned to the tank. The splash will cause water droplets to hit the glass panels. When evaporation takes place in this area, a blot of white calcium deposit is created around it. This is also true in the area where an airstone is located. As the bubbles burst, they splash onto the surrounding glass panels.

3. The most irritating and ugliest of all the water marks: when water drops from the tank to the front or sides of the tank.

We sometimes don’t notice but the splash created by filters, airstones, etc. displaces water somewhere. This gathers, and the excess trickles down to the panels of the tank. By the time you notice this, a vertical, crooked white water mark is created, showing how the water dripped. This is normally thick and rough compared to the other kinds and is very resistant to removal.

4. Water marks created by indiscriminate splashing during water changes, transferring or catching fish, or by the fish themselves.

When we do these activities, we kind mess up and water droplets find their way to the panels. If we forget to wipe these off and dry the area, these droplets evaporate and leave small, faint white water marks all over the glass panel. In my lifetime as a fishkeeper, I have learned that these are the four kinds of water marks in aquariums. They are tough to remove and definitely eyesores. It is really a struggle to keep the glass panels free of these water marks.

An Economical Solution

Many years ago I thought of using sandpaper for the purpose of removing water marks. I asked a friend who has a car restoration shop about my problem, but his opinion wasn’t what I expected. While he uses fine sandpaper when painting cars, he didn’t recommend its use on glass.

He had never used it on glass so he feared it might scratch the glass. So I never tried it. However, lately, I read that some fishkeepers used it with some success. So on that fateful day in the hardware, I decided I might as well try it and find out whether it worked for myself.

I was able buy a sheet of sandpaper with a grit size of 1200. I know that the higher the grit size, the finer it is. I have seen some up to 2000 grit size. But the highest grit size I found that day was 1200. So I settled for it.

One lazy afternoon, after doing a partial water change, I decided to try the sandpaper in one of my water mark-ridden tanks. This particular tank had all four kinds of water marks; after all, it was a 15-year-old tank bearing all the battle scars calcium could create. I cut the sand paper into quarters and soaked these in a small pail of water. This process is called wet sanding. After wetting the front panel, I began the task of rubbing the glass with the wet sandpaper.

Once in a while, I soaked the sandpaper in the pail of water. I made sure the sandpaper was always wet; otherwise, I risked scratching the glass. As I was using the sandpaper, I observed that I was able to remove the calcium deposits because the water turned creamy.

I continued with this process until I had done the whole front glass panel. After about 10 minutes of this, I washed off the front panel and wiped it dry. To my amazement, I had rid the front glass of all water marks! So I tried wiping the inside of the front glass with wet sandpaper to remove the horizontal white marks left by the waterline. These, too, disappeared.

I then focused on the white blot water marks around the filter return, and these, too, were removed. After an hour, I had my 15-year-old tank looking like new! I inspected the glass and I could not see any scratches. Either I did a really good job…or I really have very poor eyesight. I am now a firm believer in doing wet sanding using sandpaper with high grit size. In fact, nowadays, whenever I reset my old tanks, I make it a point to use sandpaper to remove the old watermarks.

When I reset them, it is like having a new tank altogether. For little hard work and 24 bucks, I get my tank looking new. You should try it.


This appeared in Animal Scene’s June 2016 issue.