Chemical pH adjusters aren't really recommended as they can cause the pH to yo-yo up and down depending on the specific composition of your tap water. pH that fluctuates rapidly is very harmful to fish. It's most likely that your water is hard as well as alkaline and for soft water fish the hardness needs reducing, which will usually lower pH at the same time as a consequence.
The only way to reduce hardness and pH safely and reliably is to mix your tap water with a proportion of RO water (pure water with no hardness. This dilution reduces hardness and pH together. We have an article on the use of RO water which may be helpful. First you need to test your tap water pH by filling a cup with cold water, testing pH immediately and then retesting after 24 hours and noting both readings. Also find out the hardness level of your tap water by checking your local water quality report on your supplier's website.
Reducing nitrates can be very simple and may just involve a series of large water changes to reduce it to an acceptable level. How often do you currently change any water, and how much each time? Test your tap water nitrate level, as it isn't really possible to reduce the tank levels below the level coming from the tap without additional filtering.
In a 180l tank you say your fish are hiding. To make them more visible plant it more densly (real or plastic). When an animal feels it can hide quickly at a moments notice you'll see it more where as in a bare tank they'll constantly hide.
I'll leave someone who does RO adjustment to talk about that ... But you do need to up your water changes, 20% minimum WEEKLY would be a normal starting point. This should keep your nitrAtes down unless your tap water is high in nitrAte. Could you test that too please? And what is current tank level? Using part RO water would also help deal with any high nitrate in tap water problems.
1)How harmful is the pH to my fish - is adjusting it necessary?
The pH in itself can be harmful if above the preferred range for the fish concerned. Most fish have an acceptable range for pH with upper and lower limits; in your case the fish you have all need to be kept in a pH below 7. A pH of 7.6 is well above the recommended maximum.
The other issue is water hardness. Fish also have an acceptable range of hardness and keeping them outside their range can have health implications. The hardness of the water has a direct impact on the fish's ability to balance the level of fluid inside it's body with that of the surrounding water. High pH usually also means hard water, and your fish all need soft water to thrive.
2) How to reduce it? Is RO water expensive and the best way?
On the assumption that you have hard water the best solution is RO water. I posted about the benefits of RO to another member recently so I'll copy that here for you:
RO water has virtually no solids of any kind and has hardness levels close to zero. If you mix it with your tap water at a suitable proportion you can adjust the hardness to the level you need.
For example, if your tap water has 16 degrees of hardness and you mix it 50:50 with RO, the resulting mix will be at 8 degrees and so on. Or you can do what I do and use 100% RO with minerals added to reach the ideal level for specific fish....that has the advantage of almost zero nitrates in the water going into the aquarium. pH will reduce too, though that doen't work in such a clear way.
How to source RO depends on your own preference: it can be bought from many LFS's for about ?2-?3 per 25 litres. If you don't need much and don't mind the inconvenience of fething it that can work.
Alternatively you can buy your own RO unit. These are best permanently plumbed in, but with suitable adapters can be run from a tap, though the process is slow and will tie a tap up for most of the day to produce sufficient RO water.
Perfectly good RO units can be bought for around ?30-?40, but prefilters and the RO membrane need replacing regularly and you will also need a TDS meter (about ?15) and a food-safe storage container to collect the water in.
If you're on metered water the cost of the water itself needs factoring in (RO membranes waste around 4/5ths of the water that goes through the filter)and if on an unmetered supply, your water supplier may need to be informed that you are installing an RO unit and can require you to have a meter.
The first step is to buy a combined kH/gH hardness test kit and test your tap water. If you post the results here we can tell you how much RO you're likely to need at a time. The API kH/gH kit is about ?8 on eBay. Do test the tap water nitrate level as well as another benefit of RO filtration is that it removes nitrates.
If you use 100% RO and remineralise it instead of mixing with tap water, another benefit is that dechlorinator is not needed as RO units also remove this and heavy metals etc.
3) What is the optimum level of water changes and how much each time?