We see in Pasuk(9:1-5 "Bring this sacrifice in front of Hashem, for on this day Hashem will appear to you."), that Moshe commanded Aharon and his sons and the Bnei Yisroel, about bringing sacrifices. However, we see at the end of the commandment(9:6 "And Moshe sadi, this is the thing that Hashem has commanded to do, and the glory of Hashem will appear to you.") that Moshe repeats again that this is a commandment from Hashem?

Why the does Moshe repeat that Hashem commanded them, and Hashem will appear to them?
The Sifra comes and explains that Moshe told the Bnei Yisroel, the same Yetzer Horah, take out of your hearts and you all should have one fear and goal to serve Hashem.
Just like Hashem is the sole creator in this world, so should your service towards Hashem be special and unique.

Comes the Gri"z (heard from Rav Goldvich tz"l, Shay Leh Torah) and explains this Sifra.
What would happen if someone would tell you, that by doing a certain Mitzvah, one would merit to see the Vilna Gaon, or the Neviim or the Avos!!
Would this Mitzvah be considered as done Lishmah, for one is doing the Mitzvah in order to attain a higher level of a spiritual level? To be closer to Hashem.
We can go even futher and ask, if one does a Mitzvah and he is doing it because he was told that by doing the Mitzvah he will merit seeing Hashem. Would we call this Mitzvah, one that was done "Loh Li Shmah"?
The answer is, yes.
For one is commanded to perform Mitzvos, for one reason and one reason only.
Because Hashem commanded him to perform the Mitzvah, and this is the will of Hashem. And not because of some external reward, even spiritual reward.
This is what the Sifra is coming to tell us.

That after Moshe told them the commandments, he then told them that "the glory of Hashem will appear to them."

After they heard this, they all gathered around and all were happy, and they started serving Hashem quickly, Moshe was afraid that maybe they were doing this because they wanted Hashem to appear to them.
Therefore Moshe told them, that evil inclination that you have in you, take out of yourselves. And serve Hashem only because that is what you were commanded to do.
Now we can understand what happened very clearly, at first Moshe told them over the commandment (9:1-4).
Yet right after he told them, he saw in Pasuk (9:5) that right away they took what Moshe commanded them and stood in front of Hashem.
When Moshe saw this, he told them to take out the Yetzer Harah from within.
Moshe told them that when one does a commandment one should do it only because of one reason, because Hashem commanded you to do it. By doing this Mitzvah, Loh Lishmah, it will bring out the true glory of Hashem.
Also, the Mitzvah is not just doing the commandment, yet it also includes the preparation for the Mitzvah. As the Parsha starts off by telling us that it was on the 8th day. Why didn't the Torah just tell us that it was the first day of the Mishkan in function?

Comes Rav David Feinstein(Kol Dodi) and explains that the Torah wants to teach us that the preparation for a Mitzvah has nearly the same importance as doing the Mitzvah itself. Just like if one helps another to perform the Mitzvah, he is also rewarded. Like someone giving a person a lift to visit the sick, or supporting Torah learning.
Same by the preparation in building the Mishkan, this is what brought out the 8th day. And it wasn't that the 8th day was the first day of the Mitzvah.
The same is by the Seder night, all the preparations, and learning the Minhagim and Halachos, is all part of the Mitzvah.

However, as we saw above, ones intention on why and how one performs a Mitzvah is very important. This we see by Nadav and Avihu. According to the Gemarah they were considered to be Gedolei Hador. However, Rashi writes that they sinned by drinking wine before entering the Mikdash.

According to Rabbi A.J. Twersky, they wanted to obtain a higher level of spirituality, and they thought that through the wine, they would free themselves, of every day tensions and pressures. They did this because they thought that this would help them serve Hashem better.
Comes the Torah and tells us that this acquiring of spirituality through an altered state of ones mind, is considered a "foreign fire", which is forbidden. He continues by explaining that spirituality is obtained only through ones hard work and dedication, by subduing ones natural inclinations in favor of Hashem's will.

There is no easy way to reach closeness to Hashem, one must work on himself with hard work.
The Chidushei Harim brings down that the main aspect of performing a Mitzvah is because Hashem has commanded us to, this is a higher level than good intentions and Kavanah.
One should keep this in mind when performing a Mitzvah. In order for one to be a real Yirei Shamayim, one must follow every aspect of Hashem's commandment, and not to do acts because one feels that this is what Hashem wants. Because this is a foreign fire.

There are Kosher fish and there are non-kosher ones.
Kosher fish have signs that show that they are Tahor.

Snapir (fins) and Kaskeses (scales). The Ramah adds that if one can not peel (by hand or utensil) them off the skin of the fish, the scales are not considered the type that are found in a kosher fish. The same is true if one peels off the scales and the whole skin comes off with it. At times it may be hard to see the Kaskasim because there are Kosher fish that loose their Kaskasim when they leave the water.(Kolbo 145:3).

Any fish that has scales, also has fins. However, if a fish has fins, one must still make sure that the fish also has scales. Without scales, one may not eat the fish. The fins are used to swim with, and the scales are found stuck around its whole body. Even if the fish doesn't have scales now, yet it will either grow them in the future, or when they leave the water they fall off, they are allowed to be eaten. Even if the fish does not have the scales on its whole body, yet only one fin and one scale, the fish is still kosher. (Rambam, Machalos Asuros, 1:24)

The Tur-Shulchan Aruch(Y.D. 83) brings down that when one finds only one scale, it should at least be found by its mouth (Lechyav) or by its tale, or under its fins. He also brings down other opinions that hold that there should be at least one scale in each of the above three places.

The Chaza"l come and bring us other signs that one can tell if a fish is kosher (the signs are in addition to fins and scales and do not replace them):

• Fish which are impure, at times are found to have a sharp forehead and no spinal cord.

• Kosher fish usually have a tail which is split into two parts, while impure ones are usually one piece.

• An impure fish gives birth like a Sheretz(swarm, insects). While a pure fish lays eggs.

Milk, Meat, Fish:

One should not eat Fish & Meat (chicken, meat) togeth, for it brings Tzoraas (Beit Yosef Y.D. 116:3)
One should also make sure not to cook fish with meat, some say not to put them together in the oven.
One is allowed to eat fish with milk, L'Halacha.
However, there are those that are more stringent and don't eat fish with butter or milk. (Beit Yosef)
While there are those, Yotsei Ashkenaz, that do eat fish with milk.

Shabbos:

One should eat fish on Shabbos for Oneg Shabbos (Shabbos 118:b)
One can not use fish skin as parchment to write a Sefer Torah, even if it is from a Kosher fish.
The Sefer "Shut Har Tzvi (O.C. 175:1)" brings down that, one who pulls a fish out of water on Shabbos, is Chayav because of "Kotzer" on Shabbos.
One is forbidden to feed fish on Shabbos, unless they are in an aquarium before Shabbos and they are dependent upon him to be fed.
(None of the above should be taken as a final Halacha, A Rav should be asked , the above is only to be "L'Hagdil Torah, U 'Lhadir.")