Is Warmer Water a Problem for Aquarium Fish During Summer Heat Waves?

 The answer is, it depends on many factors. 

1. What temperatures are natural for the fish specific you have in your tank? 
Cold water fish will be more stressed than ones from warmer climates as warm water holds less oxygen than cold water. For most tropical fish, don't worry about temperatures upto 30C  for a few weeks. 

2. Are your fish from slow or fast flowing water? 
Fish from faster flowing water will be more stressed than those who are adapted to slow flowing water as they require higher oxygen levels in the water. 

3. What is the surface area ratio of your tank? 
Tall tanks have less surface area than long tanks and therefore have lower oxygen in the water. 

4. How effective are your biological filters?
High ammonia content in your tank contribute to low oxygen as the nitrifying bacteria consumes oxygen in the nitrogen cycle. Make sure they are operating efficiently.

5. How much are you feeding? 
Feed a little less food during high heat. Just like other animals, unusual extremes of temperature cause fish to eat less. Less waste will reduce the need for bacteria conversion. Avoid major swings in food volume though or your nitrate cycle will be disrupted. 

6. Is your tank planted and with what types of plants?
Bottom-rooted plants remove more nitrates from the water and add some oxygen. 

Floating plants can block oxygen exchange at the surface and can lower oxygen unless there is high surface agitation. Removing some from your tank may help.

6. What is the volume of the tank? 
Small and large tanks are affected the most. Small tanks up to about 15 gallons due to small volume will heat up quickly as the air temperature does, they can also cool down more quickly at night. Large tanks take longer to heat up but will also hold heat longer as they take longer to cool down. 

Here's a great article for further information.
https://plecoplanet.com/pleco/aeration-and-dissolved-oxygen-in-the-aquarium/

What You Can Do to Temporarily Reduce Aquarium Water Temperature (if you don't have air conditioning)

1. Block sunlight. Close all drapes. Indirect light can heat up a room.

2. If your lights are incandescent, reduce aquarium lighting by a few hours a day. While most lights do not throw much heat, they do add some. Otherwise, maintain the usual lighting duration.  When algae and plants die, they consume oxygen in the decomposition process.

3. Do a water change with cooler water than has been aerated. Store water on a garage floor as you age it and run a bubbler in it for example. Avoid shocking the fish with too much temperature change though. 

4. Float ice packs in the water. Freeze 2L and 4 L water tight containers. Rotate them between the tank and the freezer.

5. Temporarily increase the water agitation (as long as it's not too strong for the species of fish you have). Turn on extra Hang on Back filters or air bubblers. A water pump can help, just make sure the water current is not too much for the fish in question.

6. Set up a fan to blow across the water surface. 

7. Avoid cleaning your chemical filters. Unless they are really gross, removing the nitrifying bacteria can upset the cycle and contribute to more oxygen loss.

8. Make all changes gradual (incremental). Extremes of any kind can kill your fish.

9. If heat is an ongoing challenge where you live, consider setting up cool water aquariums in your basement or on cement. That can help keep them cooler. They can be moved seasonally as well. Adding a cooling unit to your tanks might be helpful as well.

Good luck!