Aquarium leak diagnosis: telling evaporation, condensation, seam weep, and equipment failure apart

Step 0: Is there an active emergency?

Before diagnosis, check for three immediate red flags. If any apply, treat as an active leak emergency, cut power, and skip to the crack repair guide while you simultaneously work this diagnostic.

• Visible water on the floor or stand surface that was not there yesterday and the puddle is growing.
• Audible drip or trickle that you can hear from across the room.
• Water level has dropped more than 1 inch in 24 hours without recent water changes.

If none of those apply, you have time. Work the diagnostic methodically.

The diagnostic flowchart at a glance

Five major categories of water loss, ranked from most common to least.

If you have not done the methodical tests below, you cannot know which one you have. Guessing will cost you either a tank you did not need to tear apart or fish you did not need to lose.

Test 1: The marked water line test (rules in or out evaporation)

This is the first test and it eliminates the most common cause. Run it before any other diagnostic.

1. Mark the water line at the start of the test with a Sharpie on the outside glass at one corner. Use a small horizontal tick.
2. Mark the date and time next to the tick.
3. Note the room temperature and humidity if you have a sensor. A $15 ThermoPro TP49 will do.
4. Do not top off the tank during the test window.
5. Wait 24 hours, then 48 hours, then 7 days. Measure the water line each time.
6. Expected evaporation rates:

Reef tanks with sumps and protein skimmers evaporate twice as much as freshwater closed tanks of the same volume. Tanks in low-humidity rooms (below 30% RH) double the rates above. Tanks in high-humidity rooms (above 70% RH) halve them.

If your daily loss falls in the normal range for your tank type, you do not have a leak. Add a lid or install an automatic top-off (Tunze Osmolator at $130 is the standard) and move on.

If your loss exceeds the upper range by more than 25 percent, proceed to Test 2.

Test 2: Inside or outside of the glass?

Water on the outside of the glass without a falling water level means condensation, not a leak. This is the second-most-common false alarm.

1. Wipe the outside of the tank completely dry.
2. Check at 1 hour, 4 hours, 12 hours, 24 hours.
3. If water reappears uniformly on the outside (especially on the lower portions of the panels), this is condensation from warm tank water meeting cool room air. Same physics as a cold drink on a humid day.
4. If water reappears only at one specific point or along a seam line, this is a real weep or crack. Skip to Test 4.
5. If no water reappears anywhere on the outside but the water level inside is still dropping, the loss is going somewhere internally - probably through equipment plumbing into the sump or down a substrate path. Skip to Test 3.

Condensation solutions: improve room ventilation, add a fitted glass lid (drops evaporation/condensation by 70 percent), or warm the room. Condensation is not a tank problem - it is a room humidity and temperature delta problem.

Test 3: Equipment and plumbing audit

If water level is dropping but the outside of the tank is dry, the leak is in equipment plumbing. Common culprits:

• Canister filter intake or output hose connections. Fluval, Eheim, Oase canisters can dribble at the hose-to-canister fitting if the o-ring is dry, dirty, or aged. Disconnect, lubricate the o-ring with silicone grease (Dow 111, $8), reconnect.
• Sump return line bulkhead. If the tank has a drilled overflow with bulkhead fittings, the bulkhead gasket can age and weep. Drain below the bulkhead, replace the gasket (typical cost $4-8), reinstall.
• Return pump union. Hayward unions, SCH80 unions, threaded fittings. Hand-tight is not always tight enough; a moderate quarter-turn past hand-tight with a strap wrench, with fresh thread tape, usually fixes a slow weep.
• Skimmer recirculation pump on a reef sump. Often dribbles around the impeller cover after years of running. Replace the o-ring.
• HOB filter overflow. AquaClear and similar HOB filters can overflow if media is clogged. Looks like a tank leak but is actually a filter blockage.
• UV sterilizer housings. The end caps on inline UV units (Aqua UV, Pentair) weep when the bulb is replaced if the o-ring is not properly seated.

To isolate: power down each piece of equipment one at a time, leaving everything else running, and watch for 4-8 hours. The piece whose absence stops the water loss is the source.

Test 4: The paper towel test for seam weep

Seam weeps are slow. You will not see a stream; you will see a damp spot that returns minutes after you wipe it. The paper towel test surfaces these.

1. Wipe the outside of the tank completely dry. Pay extra attention to the bottom edges and corner seams.
2. Cut white paper towel strips and tape them to the outside of the tank along every seam line. Use blue painter's tape - 3M 2090 releases without leaving residue.
3. Wait 24 hours.
4. Inspect each strip. A wet spot anywhere on a strip indicates active weep at that seam location.
5. Take a photo and mark the exact spot with permanent marker on the glass.

If any strip shows wetness, you have a seam weep. Plan for a full reseam (see our silicone reseal guide). A spot-repair of one seam without redoing the others usually fails again within 6-12 months - the bead has reached end of life across the tank, not just at the visible failure point.

Test 5: Glass crack hunt with raking light

Cracks can be tiny enough to miss in normal room light. Glass cracks visible only in raking light account for many "mystery leaks".

1. Turn off room lights.
2. Hold a bright LED flashlight (Olight Baton or similar, 800+ lumens) parallel to each glass panel, lighting across the surface at a low angle.
3. Look for: a line that catches light differently than the surrounding glass, a "shimmer" or refraction shift, a thin sharp shadow.
4. Inspect every panel: front, back, both sides, bottom (laying a tank on its side temporarily is fine if you drain to 6 inches first).
5. Most cracks start at a stress concentration: bulkhead drill points, corner contact with rocks or decor, the bottom corners on rimless tanks.

If you find a crack, see our cracked glass repair procedure. Some cracks are repairable; some mean the tank is finished.

Test 6: The substrate wicking trick

This is the trickiest mode. Water leaves the tank through micro-gaps in the bottom seam or trim, runs along the substrate, and reaches the outside of the tank where it appears as a damp ring on the stand. Diagnosis is easy if you know to look for it.

1. Look for a damp ring on the stand surface that is in contact with the bottom of the tank.
2. Press a paper towel firmly against the bottom seam-corner from outside, hold for 30 seconds, inspect.
3. If the towel wicks moisture at one corner, that corner is the source.
4. Drop the tank water level below the substrate top. If wicking stops within 24 hours, substrate is the conduit.

Fix: substrate wicking from a tank with rimmed trim is usually a sign that the trim-to-glass silicone has failed at one corner. Repair requires reseaming. Substrate wicking from a rimless tank is rare and indicates a bottom-corner crack.

How fast each leak grows

Knowing the timeline tells you how urgent the response is.

• Evaporation: zero growth. Same rate week over week if conditions are stable. Not an emergency at all.
• Condensation: zero growth. Fluctuates with weather but not a leak.
• Equipment plumbing weep: variable. A stripped union goes from dribble to flood in days. A canister o-ring leak stays at the same dribble rate for months until you replace the o-ring.
• Seam weep: slow growth. A new weep is typically 1-2 ml per hour. Six months later, 5-10 ml per hour. Eighteen months later, the seam fails catastrophically. Plan reseam within 30-60 days of first detection.
• Glass crack: variable. A hairline crack may hold for years or fail tomorrow. Stop-drill any glass crack within 48 hours of identification and overlay-patch within a week.
• Substrate wicking: slow. Months of moisture transfer before stand damage. Not livestock-emergency but is house-emergency due to wood rot.

Calculating actual water loss accurately

"My tank is down 2 inches" is meaningful only if you convert to volume.

For a standard 75-gallon tank (48 in x 18 in x 21 in tall), one inch of water depth equals about 3.74 gallons. The math:

Volume per inch = (length x width) / 231 cubic inches per gallon. So a 75-gallon tank with 48 x 18 footprint has (48 x 18) / 231 = 3.74 gallons per inch of depth.

• 10 gallon (20 x 10): 0.87 gal/in
• 20 long (30 x 12): 1.56 gal/in
• 29 gallon (30 x 12): 1.56 gal/in
• 40 breeder (36 x 18): 2.81 gal/in
• 55 gallon (48 x 13): 2.70 gal/in
• 75 gallon (48 x 18): 3.74 gal/in
• 90 gallon (48 x 18): 3.74 gal/in
• 125 gallon (72 x 18): 5.61 gal/in
• 180 gallon (72 x 24): 7.48 gal/in

"Down 2 inches in a 75 gallon" is 7.5 gallons. Across one week at normal evap rates that is roughly correct. Across one day, that is a real leak.

Documentation when you suspect insurance involvement

For any leak that has caused or might cause property damage (water on flooring, sub-floor wetting, drywall staining), document before you fix.

• Photograph the leak source with the source dry, then immediately after wetting.
• Photograph the entire tank, stand, and surrounding flooring from multiple angles.
• Photograph any wet surfaces, drywall, baseboard, or carpet immediately and again 24 hours later.
• Note the date and time of first observation.
• Keep all silicone tubes, parts, and packaging until any insurance claim is resolved.
• See our aquarium insurance coverage guide for what is and is not covered under standard homeowner policies.

FAQ

Related guides

• Cracked glass repair
• Silicone reseal walkthrough
• Aquarium insurance coverage
• Livestock evacuation