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My tank temp is set at 75F and room temp is 65F, when I do a cooler water change, should I unplug my heater? If so how long should I leave the heater unplugged? thanks

That all depends... What size tank is it, what species of Corydoras, what temperature differential (or how much lower is the temp when you have done the water change), etc, etc.

I would say that you don't want to leave the heater switched off more than a few hours at that room temp - but it does depend on the fish. As Ian Fuller wrote in another post, some corys spawn at 17'C (62'F), others are perfectly happy at 28'C (82'F).

You can also turn the heater down a few degrees - that way the temperature will stay a few degrees lower, but not sink to 65'F as your room is.

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Mats

Some heaters switch off automatically when they are out of water, some don't. The heaters that don't switch off automatically can shatter when left plugged in and the water level is too low. If you don't know which type of heater you have, I'd say it's safer to unplug your heater for the time you're doing your water changes and plug it back in after the tank is full.
Amanda

Reason I asked is because I'm not certain of the cooler water change. THe heater I'm using, the lowest temp setting is 75F and that's the usual temp the tank is in. When I do 50% cooler water change using 65F water, the tank goes to around 72F and if I leave the heater on it'll go back to 75F within an hour. Is the tank supposed to stay cooler for a longer period of time? I'm trying to intiate some spawning.

Tank is a 35gal mixed cory tank, I have sterbais, trilineatus, metae, melini, duplicareaus, pymeaus, habrosus, panda and schwartzi in there.

Wow, I did a 50% cool water change after making that last post and the sterbais spawned! I just started feeding them heavily on whiteworms 2 weeks ago, cool!! Hopefully some of the others will follow

I guess I don't know why you would add water that much cooler unless you are using RO water or trying to trigger a spawn. If you have to use RO - add it slowly is a possible solution.

First, it depends what kind of heater you have as already stated. I use all submersibles myself... If it is a heater you can leave plugged in or if you are not exposing it - leave it on. The heater will bring the temp back up.

Second, it depends on how much water you change at a time and the size of your tank. On my 55s, 29s, 20 highs - I do about a palm width when I do a change. Depending on your setup, your fish, etc this can vary but it is a good general guideline for your average change. And if it is "just" a community tank with gravel and the works - once a week should be enough unless it is overcrowded.

Third item... Temperature. Unless you are trying to instigate a spawn I would say NEVER add cooler water especially at that temperature. 74-78F with a 2-4F temp drop is a great way to start an ich outbreak. I have known it forever but I appear to be a fine expert at causing that now - I just cleared out one and a half tanks that way and in the process of doing it to a third. And I know I did not cross contaminate between the first two. I had two of them on an outside wall of the house and had unplugged the heaters over the summer. The recent cold snap dropped the temp on them about 4 degrees overnight - welcome the ich. One outbreak in a tank that hasn't seen it in years and with fish that have been in it for a minimum of one year - no kidding. This tank has been up and running 3 years and no disease since the first 6 months... And the other full of young BN from the first tank - say good bye to 400 young BN. Now tonight, it appears the tank next door to that one has fallen ill (that has to be a cross contamination the tanks are too close) and I have sucked out 30 1" BN already today in the first day. I haven't had ich other then in some freshly bought Clown loaches in over 5 years... It's been so long since I have seen it I didn't realize what was going on at first - I honestly thought my first tank crashed and it was ammonia or something. It is making me sick. As a translation I have flushed about $600 (to me as a breeder selling to LFS) in fish the past week - retail it translates to more like $3000. Putting it that way, I am now fully ill...

Yes I was trying to induce the corys to spawn and it did the trick!
Here's the female sterbai carrying her eggs in her pelvic fins

That's great... I have been working with them myself recently. Fattening up mine for another run at a nice hatch right now.

BTW... I don't know your level of experience or ambitions... But if you have an empty take you can put a bunch of slate down in it for them to spawn on and then collect the eggs. It works a lot better for maximizing your hatch. I didnt think mine were old enough to spawn when they did. Collected about 30 eggs and proceeded to only keep 6 alive (vacation right after they went free swimming.) But I am ready now...

Very new to breeding corys, this will be my second spawning, first being the bronze. I only managed to retrieve around 15 eggs and 5 were infertile. The rest still haven't hatched yet but they are an amber colour so I'm hopeful that they will hatch tommorrow.

Yeah I need to move them over to their own tank, so I can retrieve the eggs easier.

She spawned again, about a week later from the last time. I did not do a cooler water change to initiate this spawning, temp was steady at 75F (24C), guess once they start, a cooler water change is not necessary to trigger them.

This time I was able to collect around 30 eggs. Another reason to move them to thier own tank, the other corys ate a few of the eggs before I could collect them