There are lots of general plant feeds and fewer orchid feeds available, personally I would only recommend two, Akernes Rain Mix which is a powder feed or Focus grow/bloom which is a liquid feed.
You will see lots of differing views on feeding orchids, I think the best feed is Akernes rain mix but there are many available. I would recommend a feed that contains all the main elements of N-P-K, with P being the lowest ratio. The feed should include trace elements including Calcium and magnesium especially if you choose to use rain water or RO as these are lacking these important elements. We use Akerne’s Rain Mix and think you will find it very difficult to beat for quality and value compared to a lot of liquid plant feeds available. Some Calcium and Magnesium is available in tap water so not as much as a issue if you choose tap water. But every ones tap water is different so you can check with your local water authority to find out what is the make up of your tap water. I use RO (reverse osmosis) as it is pure water so is very clean and always available, I did use rain water but didn’t like how you need to store a large amount which does get very dirty in a short time if not used. And there was always that moment in summer when the water dried up! Whereas with RO you can have a smaller storage container, I use 120 litres at a time and as I feed at every watering there is no messing about with some plants needing a flush or not. I don't think anyone would recommend using tap water for your orchids, but in some regions it can be low in total dissolved solids so ok for orchids.
For those that simply have no choice to use tap water, then it would be wise to use an orchid feed but at a lesser strength than recommended, so maybe at 1/4 or 1/2 strength.
I check the pH of the water after I have mixed my feed to water and aim in the range of 5.5 – 6.5 which suits the majority of orchids, this is important as either side of this range and the solution becomes toxic resulting in important elements of the feed being locked out thus causing deficiencies. Feed strength is important too and can be tested with a conductivity pen, strength can be altered to suit your plants but I would recommend feeding around 65 ppm N approx 500-550 us/m at every watering or ¼ strength of the recommended dosage of a standard feed . I like to feed at every watering at this strength so it is a constant liquid feed regime and because the strength is not high, have found no problems with dissolved solids building up which can cause root burn so eliminates the need to flush the pots with pure water. My orchids do however get pure water sometimes when I get lazy and haven’t had time to mix up and adjust my feed, but this is very rarely done. The feed strengths can be altered to suit your specific conditions as the conditions in the UK are very different from someone growing in the southern hemisphere, I know growers in the USA that feed all there plants at 125 ppm N but expect all aspects of growing are increased like day length and light thus this will increase the plants need for more. This higher strength could possibly cause harm to the more delicate species so I think it is sensible to feed at a strength all your plants can tolerate, at Akernes rain mix recomended feed strength we have noticed a significant increase in growth and flowers. So you could say there is no need to increase above this level and doing so is pouring excess feed down the drain!
A lot of growers state you don’t feed in winter, I don’t agree with this theory and would say if there is a need to water then there is a need to feed. In most circumstances in winter you would expect the watering frequencies to be less than in summer, thus the feeding will be too as the total frequencies are less. For example; in summer I may water up to three times a week, in winter it may be once every week or two. So you can see that even though I feed at every watering the plants do get a significant feed reduction in winter, cool growing Dendrobiums and Cattleyas get far less if anything at all as they only get a light spraying to prevent excessive shrivelling or dessication.
Orchids like Phalaenopsis, Masdevallias and Paphiopedilum continually grow so don’t have the ‘so called rest’ so will get far more feed/watering. My philosophy is all orchids will get the so called rest in their own right, as at some point in the year(winter) everything will be reduced in some way; light, day length, frequency of water and feed.
As with all feeds you need to keep the lids tightly closed after use to prevent spoiling, powder feeds absorb moisture like sugar does, liquid feeds evapourate and can crystalize so both need to be kept air tight. Also don’t put any wet or damp measuring spoons in with a powder feed as the feed will absorb this moisture and turn the powder solid. Liquid feed will need a good shake to make sure any sediment or crystals are thoroughly mixed.
Most orchid feeds once added to rain or RO water should fall in the acceptable pH range, but if your rain or RO water is too acidic then it can fall below. For example; RO membranes deteriote resulting in the water pH to decrease, this is why it is wise to check the quality and pH of your chosen water.
On the whole trusting the two mentioned orchid feeds and adding it to quality rain or RO water should reap great results without too much fuss!