Talk to me about stray voltage, please!

I've had an ongoing problem with harlequin rasboras dying one at a time, and I think it may be from stray voltage. Tell me what you think:

• The only symptom shared by all fish is an abrupt change in behavior. The fish begins twitching its fins, swims repeatedly around and around the same area of the tank, ignores other fish, stops feeding (due to lack of interest or inability?), and eventually loses buoyancy control.
• The fish may die within hours, or go slowly downhill until I euthanize it.
• I'm not seeing HITH, body wounds, bloody streaks, popeye, dropsy, fin damage, or parasites.
• What I thought might be a fungal/bacterial infection turned out to be (I believe) a heavy slime coat and graying out loss of color.
• Treatments with Maracyn Plus (main tank) and Seachem ParaGuard (QT tank) have had no noticeable effect. Seemingly healthy fish fell ill and died during the treatments.
• MT snails and amano shrimp are perfectly healthy and happy.
• Water params are fine, testing repeatedly with API liquid kit.
• All deaths have occurred in the hours/days right after water changes — during which I usually unplug/replug HOBs and heaters.

So ... Does that sound like stray voltage??

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Main tank: Marina Slim HOB (motor in the tank), Via Aqua heater, battery-operated temperature probe, dual All-Glass fixture (on glass lid, not open tank).

QT tank: Nano HOB, Cascade heater, no light.

Both tanks plugged into the only available outlet, which is not grounded. I'm looking at fixing that but in the meantime ... Will adding a GFIC plug/powerstrip do any good on an ungrounded outlet? Will changing to a canister filter and inline heater (which I'd like to do for a variety of reasons) end the problem??

SubRosa said:

If you want to know if there's stray voltage in your tank there's an easy test and a different more accurate one as well. For the accurate test get a voltmeter, stick one probe in the tank and touch the other to a good ground. Read the voltage. The easy way is to stick a hand into the tank that has a small cut or a hangnail or some other break in the skin. If there's voltage in the water you won't need a meter to find out!

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Well, I haven't felt any tingles/buzzing ... I know what that feels like from problems with an old internal filter that I used briefly a year ago. As for voltmeter/multimeter, I'd have to find a model with 15-20 foot leads, as the nearest grounded outlets are in other rooms.

GCL said:

Two thoughts come to mind;
Perhaps your tap water has changed somehow? Are you adding anything in particular to the water before you add it back into your tank?
This doesn't sound good to me and I hope someone who has experience with these symptoms will chime in to help. I hope you can figure this out so as to stop your losses.

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No changes in tap water. Same water goes into my betta's tank, and she's fine. I'm planning to replace the substrate in the 20g long, and possibly upgrade to a 29g at the same time. I will keep some of the rocks, the same driftwood (might boil it), all the plants, and my two shrimp. I'm down to two harlies. I won't try adding fish again until after the rescape.

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Still need to know: Can stray voltage affect a tank with an external canister filter/inline heater?